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Update for Coq trunk version intended for final 8.4 release
author Adam Chlipala <adam@chlipala.net>
date Tue, 17 Jul 2012 16:37:58 -0400
parents 5986e9fd40b5
children 686cf945dd02
rev   line source
adam@398 1 (* Copyright (c) 2008-2012, Adam Chlipala
adamc@2 2 *
adamc@2 3 * This work is licensed under a
adamc@2 4 * Creative Commons Attribution-Noncommercial-No Derivative Works 3.0
adamc@2 5 * Unported License.
adamc@2 6 * The license text is available at:
adamc@2 7 * http://creativecommons.org/licenses/by-nc-nd/3.0/
adamc@2 8 *)
adamc@2 9
adamc@3 10 (* begin hide *)
adam@312 11 Require Import Bool Arith List.
adamc@2 12
adam@314 13 Require Import CpdtTactics.
adamc@14 14
adamc@14 15 Set Implicit Arguments.
adamc@3 16 (* end hide *)
adamc@2 17
adamc@2 18
adamc@25 19 (** %\chapter{Some Quick Examples}% *)
adamc@25 20
adamc@25 21
adam@399 22 (** I will start off by jumping right in to a fully worked set of examples, building certified compilers from increasingly complicated source languages to stack machines. We will meet a few useful tactics and see how they can be used in manual proofs, and we will also see how easily these proofs can be automated instead. This chapter is not meant to give full explanations of the features that are employed. Rather, it is meant more as an advertisement of what is possible. Later chapters will introduce all of the concepts in bottom-up fashion.
adam@279 23
adam@314 24 As always, you can step through the source file %\texttt{%#<tt>#StackMachine.v#</tt>#%}% for this chapter interactively in Proof General. Alternatively, to get a feel for the whole lifecycle of creating a Coq development, you can enter the pieces of source code in this chapter in a new %\texttt{%#<tt>#.v#</tt>#%}% file in an Emacs buffer. If you do the latter, include two lines
adam@314 25
adam@399 26 %\index{Vernacular commands!Require}%[Require Import Bool Arith List CpdtTactics.]
adam@314 27
adam@314 28 %\noindent{}%and
adam@314 29
adam@399 30 %\index{Vernacular commands!Set Implicit Arguments}%[Set Implicit Arguments.]
adam@314 31
adam@399 32 %\noindent{}%at the start of the file, to match some code hidden in this rendering of the chapter source. In general, similar commands will be hidden in the book rendering of each chapter's source code, so you will need to insert them in from-scratch replayings of the code that is presented. To be more specific, every chapter begins with some imports of other modules, followed by [Set Implicit Arguments.], where the latter affects the default behavior of definitions regarding type inference.
adam@307 33 *)
adamc@9 34
adamc@9 35
adamc@20 36 (** * Arithmetic Expressions Over Natural Numbers *)
adamc@2 37
adamc@40 38 (** We will begin with that staple of compiler textbooks, arithmetic expressions over a single type of numbers. *)
adamc@9 39
adamc@20 40 (** ** Source Language *)
adamc@9 41
adam@311 42 (** We begin with the syntax of the source language.%\index{Vernacular commands!Inductive}% *)
adamc@2 43
adamc@4 44 Inductive binop : Set := Plus | Times.
adamc@2 45
adam@311 46 (** Our first line of Coq code should be unsurprising to ML and Haskell programmers. We define an %\index{algebraic datatypes}%algebraic datatype [binop] to stand for the binary operators of our source language. There are just two wrinkles compared to ML and Haskell. First, we use the keyword [Inductive], in place of %\texttt{%#<tt>#data#</tt>#%}%, %\texttt{%#<tt>#datatype#</tt>#%}%, or %\texttt{%#<tt>#type#</tt>#%}%. This is not just a trivial surface syntax difference; inductive types in Coq are much more expressive than garden variety algebraic datatypes, essentially enabling us to encode all of mathematics, though we begin humbly in this chapter. Second, there is the %\index{Gallina terms!Set}%[: Set] fragment, which declares that we are defining a datatype that should be thought of as a constituent of programs. Later, we will see other options for defining datatypes in the universe of proofs or in an infinite hierarchy of universes, encompassing both programs and proofs, that is useful in higher-order constructions. *)
adamc@9 47
adamc@4 48 Inductive exp : Set :=
adamc@4 49 | Const : nat -> exp
adamc@4 50 | Binop : binop -> exp -> exp -> exp.
adamc@2 51
adamc@9 52 (** Now we define the type of arithmetic expressions. We write that a constant may be built from one argument, a natural number; and a binary operation may be built from a choice of operator and two operand expressions.
adamc@9 53
adam@417 54 A note for readers following along in the PDF version: %\index{coqdoc}%coqdoc supports pretty-printing of tokens in %\LaTeX{}%#LaTeX# or HTML. Where you see a right arrow character, the source contains the ASCII text %\texttt{%#<tt>#->#</tt>#%}%. Other examples of this substitution appearing in this chapter are a double right arrow for %\texttt{%#<tt>#=>#</tt>#%}%, the inverted %`%#'#A' symbol for %\texttt{%#<tt>#forall#</tt>#%}%, and the Cartesian product %`%#'#X' for %\texttt{%#<tt>#*#</tt>#%}%. When in doubt about the ASCII version of a symbol, you can consult the chapter source code.
adamc@9 55
adamc@9 56 %\medskip%
adamc@9 57
adam@311 58 Now we are ready to say what these programs mean. We will do this by writing an %\index{interpreters}%interpreter that can be thought of as a trivial operational or denotational semantics. (If you are not familiar with these semantic techniques, no need to worry; we will stick to %``%#"#common sense#"#%''% constructions.)%\index{Vernacular commands!Definition}% *)
adamc@9 59
adamc@4 60 Definition binopDenote (b : binop) : nat -> nat -> nat :=
adamc@4 61 match b with
adamc@4 62 | Plus => plus
adamc@4 63 | Times => mult
adamc@4 64 end.
adamc@2 65
adamc@9 66 (** The meaning of a binary operator is a binary function over naturals, defined with pattern-matching notation analogous to the %\texttt{%#<tt>#case#</tt>#%}% and %\texttt{%#<tt>#match#</tt>#%}% of ML and Haskell, and referring to the functions [plus] and [mult] from the Coq standard library. The keyword [Definition] is Coq's all-purpose notation for binding a term of the programming language to a name, with some associated syntactic sugar, like the notation we see here for defining a function. That sugar could be expanded to yield this definition:
adamc@9 67 [[
adamc@9 68 Definition binopDenote : binop -> nat -> nat -> nat := fun (b : binop) =>
adamc@9 69 match b with
adamc@9 70 | Plus => plus
adamc@9 71 | Times => mult
adamc@9 72 end.
adamc@205 73 ]]
adamc@205 74
adamc@9 75 In this example, we could also omit all of the type annotations, arriving at:
adamc@9 76 [[
adamc@9 77 Definition binopDenote := fun b =>
adamc@9 78 match b with
adamc@9 79 | Plus => plus
adamc@9 80 | Times => mult
adamc@9 81 end.
adamc@205 82 ]]
adamc@205 83
adam@398 84 Languages like Haskell and ML have a convenient %\index{principal types}\index{type inference}%_principal types_ property, which gives us strong guarantees about how effective type inference will be. Unfortunately, Coq's type system is so expressive that any kind of %``%#"#complete#"#%''% type inference is impossible, and the task even seems to be hard heuristically in practice. Nonetheless, Coq includes some very helpful heuristics, many of them copying the workings of Haskell and ML type-checkers for programs that fall in simple fragments of Coq's language.
adamc@9 85
adam@399 86 This is as good a time as any to mention the preponderance of different languages associated with Coq. The theoretical foundation of Coq is a formal system called the%\index{Calculus of Inductive Constructions}\index{CIC|see{Calculus of Inductive Constructions}}% _Calculus of Inductive Constructions_ (CIC)%~\cite{CIC}%, which is an extension of the older%\index{Calculus of Constructions}\index{CoC|see{Calculus of Constructions}}% _Calculus of Constructions_ (CoC)%~\cite{CoC}%. CIC is quite a spartan foundation, which is helpful for proving metatheory but not so helpful for real development. Still, it is nice to know that it has been proved that CIC enjoys properties like%\index{strong normalization}% _strong normalization_ %\cite{CIC}%, meaning that every program (and, more importantly, every proof term) terminates; and%\index{relative consistency}% _relative consistency_ %\cite{SetsInTypes}% with systems like versions of %\index{Zermelo-Fraenkel set theory}%Zermelo-Fraenkel set theory, which roughly means that you can believe that Coq proofs mean that the corresponding propositions are %``%#"#really true,#"#%''% if you believe in set theory.
adamc@9 87
adam@399 88 Coq is actually based on an extension of CIC called%\index{Gallina}% _Gallina_. The text after the [:=] and before the period in the last code example is a term of Gallina. Gallina adds many useful features that are not compiled internally to more primitive CIC features. The important metatheorems about CIC have not been extended to the full breadth of these features, but most Coq users do not seem to lose much sleep over this omission.
adamc@9 89
adam@399 90 Next, there is%\index{Ltac}% _Ltac_, Coq's domain-specific language for writing proofs and decision procedures. We will see some basic examples of Ltac later in this chapter, and much of this book is devoted to more involved Ltac examples.
adamc@9 91
adam@399 92 Finally, commands like [Inductive] and [Definition] are part of%\index{Vernacular commands}% _the Vernacular_, which includes all sorts of useful queries and requests to the Coq system. Every Coq source file is a series of vernacular commands, where many command forms take arguments that are Gallina or Ltac programs. (Actually, Coq source files are more like _trees_ of vernacular commands, thanks to various nested scoping constructs.)
adamc@9 93
adamc@9 94 %\medskip%
adamc@9 95
adam@311 96 We can give a simple definition of the meaning of an expression:%\index{Vernacular commands!Fixpoint}% *)
adamc@9 97
adamc@4 98 Fixpoint expDenote (e : exp) : nat :=
adamc@4 99 match e with
adamc@4 100 | Const n => n
adamc@4 101 | Binop b e1 e2 => (binopDenote b) (expDenote e1) (expDenote e2)
adamc@4 102 end.
adamc@2 103
adamc@9 104 (** We declare explicitly that this is a recursive definition, using the keyword [Fixpoint]. The rest should be old hat for functional programmers. *)
adamc@2 105
adam@398 106 (** It is convenient to be able to test definitions before starting to prove things about them. We can verify that our semantics is sensible by evaluating some sample uses, using the command %\index{Vernacular commands!Eval}%[Eval]. This command takes an argument expressing a %\index{reduction strategy}%_reduction strategy_, or an %``%#"#order of evaluation.#"#%''% Unlike with ML, which hardcodes an _eager_ reduction strategy, or Haskell, which hardcodes a _lazy_ strategy, in Coq we are free to choose between these and many other orders of evaluation, because all Coq programs terminate. In fact, Coq silently checked %\index{termination checking}%termination of our [Fixpoint] definition above, using a simple heuristic based on monotonically decreasing size of arguments across recursive calls. Specifically, recursive calls must be made on arguments that were pulled out of the original recursive argument with [match] expressions. (In Chapter 7, we will see some ways of getting around this restriction, though simply removing the restriction would leave Coq useless as a theorem proving tool, for reasons we will start to learn about in the next chapter.)
adam@311 107
adam@311 108 To return to our test evaluations, we run the [Eval] command using the [simpl] evaluation strategy, whose definition is best postponed until we have learned more about Coq's foundations, but which usually gets the job done. *)
adamc@16 109
adamc@16 110 Eval simpl in expDenote (Const 42).
adamc@205 111 (** [= 42 : nat] *)
adamc@205 112
adamc@16 113 Eval simpl in expDenote (Binop Plus (Const 2) (Const 2)).
adamc@205 114 (** [= 4 : nat] *)
adamc@205 115
adamc@16 116 Eval simpl in expDenote (Binop Times (Binop Plus (Const 2) (Const 2)) (Const 7)).
adamc@205 117 (** [= 28 : nat] *)
adamc@9 118
adamc@20 119 (** ** Target Language *)
adamc@4 120
adamc@10 121 (** We will compile our source programs onto a simple stack machine, whose syntax is: *)
adamc@2 122
adamc@4 123 Inductive instr : Set :=
adam@311 124 | iConst : nat -> instr
adam@311 125 | iBinop : binop -> instr.
adamc@2 126
adamc@4 127 Definition prog := list instr.
adamc@4 128 Definition stack := list nat.
adamc@2 129
adamc@10 130 (** An instruction either pushes a constant onto the stack or pops two arguments, applies a binary operator to them, and pushes the result onto the stack. A program is a list of instructions, and a stack is a list of natural numbers.
adamc@10 131
adam@311 132 We can give instructions meanings as functions from stacks to optional stacks, where running an instruction results in [None] in case of a stack underflow and results in [Some s'] when the result of execution is the new stack [s']. %\index{Gallina operators!::}%The infix operator [::] is %``%#"#list cons#"#%''% from the Coq standard library.%\index{Gallina terms!option}% *)
adamc@10 133
adamc@4 134 Definition instrDenote (i : instr) (s : stack) : option stack :=
adamc@4 135 match i with
adam@311 136 | iConst n => Some (n :: s)
adam@311 137 | iBinop b =>
adamc@4 138 match s with
adamc@4 139 | arg1 :: arg2 :: s' => Some ((binopDenote b) arg1 arg2 :: s')
adamc@4 140 | _ => None
adamc@4 141 end
adamc@4 142 end.
adamc@2 143
adam@311 144 (** With [instrDenote] defined, it is easy to define a function [progDenote], which iterates application of [instrDenote] through a whole program. *)
adamc@206 145
adamc@206 146 Fixpoint progDenote (p : prog) (s : stack) : option stack :=
adamc@206 147 match p with
adamc@206 148 | nil => Some s
adamc@206 149 | i :: p' =>
adamc@206 150 match instrDenote i s with
adamc@206 151 | None => None
adamc@206 152 | Some s' => progDenote p' s'
adamc@206 153 end
adamc@206 154 end.
adamc@2 155
adamc@4 156
adamc@9 157 (** ** Translation *)
adamc@4 158
adam@311 159 (** Our compiler itself is now unsurprising. The list concatenation operator %\index{Gallina operators!++}%[++] comes from the Coq standard library. *)
adamc@2 160
adamc@4 161 Fixpoint compile (e : exp) : prog :=
adamc@4 162 match e with
adam@311 163 | Const n => iConst n :: nil
adam@311 164 | Binop b e1 e2 => compile e2 ++ compile e1 ++ iBinop b :: nil
adamc@4 165 end.
adamc@2 166
adamc@2 167
adamc@16 168 (** Before we set about proving that this compiler is correct, we can try a few test runs, using our sample programs from earlier. *)
adamc@16 169
adamc@16 170 Eval simpl in compile (Const 42).
adam@311 171 (** [= iConst 42 :: nil : prog] *)
adamc@206 172
adamc@16 173 Eval simpl in compile (Binop Plus (Const 2) (Const 2)).
adam@311 174 (** [= iConst 2 :: iConst 2 :: iBinop Plus :: nil : prog] *)
adamc@206 175
adamc@16 176 Eval simpl in compile (Binop Times (Binop Plus (Const 2) (Const 2)) (Const 7)).
adam@311 177 (** [= iConst 7 :: iConst 2 :: iConst 2 :: iBinop Plus :: iBinop Times :: nil : prog] *)
adamc@16 178
adamc@40 179 (** We can also run our compiled programs and check that they give the right results. *)
adamc@16 180
adamc@16 181 Eval simpl in progDenote (compile (Const 42)) nil.
adamc@206 182 (** [= Some (42 :: nil) : option stack] *)
adamc@206 183
adamc@16 184 Eval simpl in progDenote (compile (Binop Plus (Const 2) (Const 2))) nil.
adamc@206 185 (** [= Some (4 :: nil) : option stack] *)
adamc@206 186
adam@311 187 Eval simpl in progDenote (compile (Binop Times (Binop Plus (Const 2) (Const 2))
adam@311 188 (Const 7))) nil.
adamc@206 189 (** [= Some (28 :: nil) : option stack] *)
adamc@16 190
adamc@16 191
adamc@20 192 (** ** Translation Correctness *)
adamc@4 193
adam@311 194 (** We are ready to prove that our compiler is implemented correctly. We can use a new vernacular command [Theorem] to start a correctness proof, in terms of the semantics we defined earlier:%\index{Vernacular commands!Theorem}% *)
adamc@11 195
adamc@26 196 Theorem compile_correct : forall e, progDenote (compile e) nil = Some (expDenote e :: nil).
adamc@11 197 (* begin hide *)
adamc@11 198 Abort.
adamc@11 199 (* end hide *)
adamc@22 200 (* begin thide *)
adamc@11 201
adam@399 202 (** Though a pencil-and-paper proof might clock out at this point, writing %``%#"#by a routine induction on [e],#"#%''% it turns out not to make sense to attack this proof directly. We need to use the standard trick of%\index{strengthening the induction hypothesis}% _strengthening the induction hypothesis_. We do that by proving an auxiliary lemma, using the command [Lemma] that is a synonym for [Theorem], conventionally used for less important theorems that appear in the proofs of primary theorems.%\index{Vernacular commands!Lemma}% *)
adamc@2 203
adamc@206 204 Lemma compile_correct' : forall e p s,
adamc@206 205 progDenote (compile e ++ p) s = progDenote p (expDenote e :: s).
adamc@11 206
adam@399 207 (** After the period in the [Lemma] command, we are in%\index{interactive proof-editing mode}% _the interactive proof-editing mode_. We find ourselves staring at this ominous screen of text:
adamc@11 208
adamc@11 209 [[
adamc@11 210 1 subgoal
adamc@11 211
adamc@11 212 ============================
adamc@15 213 forall (e : exp) (p : list instr) (s : stack),
adamc@15 214 progDenote (compile e ++ p) s = progDenote p (expDenote e :: s)
adamc@206 215
adamc@11 216 ]]
adamc@11 217
adam@311 218 Coq seems to be restating the lemma for us. What we are seeing is a limited case of a more general protocol for describing where we are in a proof. We are told that we have a single subgoal. In general, during a proof, we can have many pending %\index{subgoals}%subgoals, each of which is a logical proposition to prove. Subgoals can be proved in any order, but it usually works best to prove them in the order that Coq chooses.
adamc@11 219
adam@311 220 Next in the output, we see our single subgoal described in full detail. There is a double-dashed line, above which would be our free variables and %\index{hypotheses}%hypotheses, if we had any. Below the line is the %\index{conclusion}%conclusion, which, in general, is to be proved from the hypotheses.
adamc@11 221
adam@399 222 We manipulate the proof state by running commands called%\index{tactics}% _tactics_. Let us start out by running one of the most important tactics:%\index{tactics!induction}%
adamc@11 223 *)
adamc@11 224
adamc@4 225 induction e.
adamc@2 226
adamc@11 227 (** We declare that this proof will proceed by induction on the structure of the expression [e]. This swaps out our initial subgoal for two new subgoals, one for each case of the inductive proof:
adamc@11 228
adam@311 229 %\vspace{.1in} \noindent 2 \coqdockw{subgoals}\vspace{-.1in}%#<tt>2 subgoals</tt>#
adam@311 230
adamc@11 231 [[
adamc@11 232 n : nat
adamc@11 233 ============================
adamc@11 234 forall (s : stack) (p : list instr),
adamc@11 235 progDenote (compile (Const n) ++ p) s =
adamc@11 236 progDenote p (expDenote (Const n) :: s)
adamc@11 237 ]]
adam@311 238 %\noindent \coqdockw{subgoal} 2 \coqdockw{is}:%#<tt>subgoal 2 is</tt>#
adamc@11 239 [[
adamc@11 240 forall (s : stack) (p : list instr),
adamc@11 241 progDenote (compile (Binop b e1 e2) ++ p) s =
adamc@11 242 progDenote p (expDenote (Binop b e1 e2) :: s)
adamc@206 243
adamc@11 244 ]]
adamc@11 245
adam@311 246 The first and current subgoal is displayed with the double-dashed line below free variables and hypotheses, while later subgoals are only summarized with their conclusions. We see an example of a %\index{free variable}%free variable in the first subgoal; [n] is a free variable of type [nat]. The conclusion is the original theorem statement where [e] has been replaced by [Const n]. In a similar manner, the second case has [e] replaced by a generalized invocation of the [Binop] expression constructor. We can see that proving both cases corresponds to a standard proof by %\index{structural induction}%structural induction.
adamc@11 247
adam@311 248 We begin the first case with another very common tactic.%\index{tactics!intros}%
adamc@11 249 *)
adamc@11 250
adamc@4 251 intros.
adamc@11 252
adamc@11 253 (** The current subgoal changes to:
adamc@11 254 [[
adamc@11 255
adamc@11 256 n : nat
adamc@11 257 s : stack
adamc@11 258 p : list instr
adamc@11 259 ============================
adamc@11 260 progDenote (compile (Const n) ++ p) s =
adamc@11 261 progDenote p (expDenote (Const n) :: s)
adamc@206 262
adamc@11 263 ]]
adamc@11 264
adamc@11 265 We see that [intros] changes [forall]-bound variables at the beginning of a goal into free variables.
adamc@11 266
adam@311 267 To progress further, we need to use the definitions of some of the functions appearing in the goal. The [unfold] tactic replaces an identifier with its definition.%\index{tactics!unfold}%
adamc@11 268 *)
adamc@11 269
adamc@4 270 unfold compile.
adamc@11 271 (** [[
adamc@11 272 n : nat
adamc@11 273 s : stack
adamc@11 274 p : list instr
adamc@11 275 ============================
adam@311 276 progDenote ((iConst n :: nil) ++ p) s =
adamc@11 277 progDenote p (expDenote (Const n) :: s)
adamc@206 278
adam@302 279 ]]
adam@302 280 *)
adamc@11 281
adamc@4 282 unfold expDenote.
adamc@11 283 (** [[
adamc@11 284 n : nat
adamc@11 285 s : stack
adamc@11 286 p : list instr
adamc@11 287 ============================
adam@311 288 progDenote ((iConst n :: nil) ++ p) s = progDenote p (n :: s)
adamc@206 289
adamc@11 290 ]]
adamc@11 291
adam@311 292 We only need to unfold the first occurrence of [progDenote] to prove the goal. An [at] clause used with [unfold] specifies a particular occurrence of an identifier to unfold, where we count occurrences from left to right.%\index{tactics!unfold}% *)
adamc@11 293
adamc@11 294 unfold progDenote at 1.
adamc@11 295 (** [[
adamc@11 296 n : nat
adamc@11 297 s : stack
adamc@11 298 p : list instr
adamc@11 299 ============================
adamc@11 300 (fix progDenote (p0 : prog) (s0 : stack) {struct p0} :
adamc@11 301 option stack :=
adamc@11 302 match p0 with
adamc@11 303 | nil => Some s0
adamc@11 304 | i :: p' =>
adamc@11 305 match instrDenote i s0 with
adamc@11 306 | Some s' => progDenote p' s'
adamc@11 307 | None => None (A:=stack)
adamc@11 308 end
adam@311 309 end) ((iConst n :: nil) ++ p) s =
adamc@11 310 progDenote p (n :: s)
adamc@206 311
adamc@11 312 ]]
adamc@11 313
adam@399 314 This last [unfold] has left us with an anonymous fixpoint version of [progDenote], which will generally happen when unfolding recursive definitions. Note that Coq has automatically renamed the [fix] arguments [p] and [s] to [p0] and [s0], to avoid clashes with our local free variables. There is also a subterm [None (A:=stack)], which has an annotation specifying that the type of the term ought to be [option A]. This is phrased as an explicit instantiation of a named type parameter [A] from the definition of [option].
adam@311 315
adam@311 316 Fortunately, in this case, we can eliminate the complications of anonymous recursion right away, since the structure of the argument ([iConst n :: nil) ++ p] is known, allowing us to simplify the internal pattern match with the [simpl] tactic, which applies the same reduction strategy that we used earlier with [Eval] (and whose details we still postpone).%\index{tactics!simpl}%
adamc@11 317 *)
adamc@11 318
adamc@4 319 simpl.
adamc@11 320 (** [[
adamc@11 321 n : nat
adamc@11 322 s : stack
adamc@11 323 p : list instr
adamc@11 324 ============================
adamc@11 325 (fix progDenote (p0 : prog) (s0 : stack) {struct p0} :
adamc@11 326 option stack :=
adamc@11 327 match p0 with
adamc@11 328 | nil => Some s0
adamc@11 329 | i :: p' =>
adamc@11 330 match instrDenote i s0 with
adamc@11 331 | Some s' => progDenote p' s'
adamc@11 332 | None => None (A:=stack)
adamc@11 333 end
adamc@11 334 end) p (n :: s) = progDenote p (n :: s)
adamc@206 335
adamc@11 336 ]]
adamc@11 337
adam@311 338 Now we can unexpand the definition of [progDenote]:%\index{tactics!fold}%
adamc@11 339 *)
adamc@11 340
adamc@11 341 fold progDenote.
adamc@11 342 (** [[
adamc@11 343 n : nat
adamc@11 344 s : stack
adamc@11 345 p : list instr
adamc@11 346 ============================
adamc@11 347 progDenote p (n :: s) = progDenote p (n :: s)
adamc@206 348
adamc@11 349 ]]
adamc@11 350
adam@311 351 It looks like we are at the end of this case, since we have a trivial equality. Indeed, a single tactic finishes us off:%\index{tactics!reflexivity}%
adamc@11 352 *)
adamc@11 353
adamc@4 354 reflexivity.
adamc@2 355
adamc@11 356 (** On to the second inductive case:
adamc@11 357
adamc@11 358 [[
adamc@11 359 b : binop
adamc@11 360 e1 : exp
adamc@11 361 IHe1 : forall (s : stack) (p : list instr),
adamc@11 362 progDenote (compile e1 ++ p) s = progDenote p (expDenote e1 :: s)
adamc@11 363 e2 : exp
adamc@11 364 IHe2 : forall (s : stack) (p : list instr),
adamc@11 365 progDenote (compile e2 ++ p) s = progDenote p (expDenote e2 :: s)
adamc@11 366 ============================
adamc@11 367 forall (s : stack) (p : list instr),
adamc@11 368 progDenote (compile (Binop b e1 e2) ++ p) s =
adamc@11 369 progDenote p (expDenote (Binop b e1 e2) :: s)
adamc@206 370
adamc@11 371 ]]
adamc@11 372
adam@311 373 We see our first example of %\index{hypotheses}%hypotheses above the double-dashed line. They are the inductive hypotheses [IHe1] and [IHe2] corresponding to the subterms [e1] and [e2], respectively.
adamc@11 374
adam@399 375 We start out the same way as before, introducing new free variables and unfolding and folding the appropriate definitions. The seemingly frivolous [unfold]/[fold] pairs are actually accomplishing useful work, because [unfold] will sometimes perform easy simplifications. %\index{tactics!intros}\index{tactics!unfold}\index{tactics!fold}% *)
adamc@11 376
adamc@4 377 intros.
adamc@4 378 unfold compile.
adamc@4 379 fold compile.
adamc@4 380 unfold expDenote.
adamc@4 381 fold expDenote.
adamc@11 382
adamc@44 383 (** Now we arrive at a point where the tactics we have seen so far are insufficient. No further definition unfoldings get us anywhere, so we will need to try something different.
adamc@11 384
adamc@11 385 [[
adamc@11 386 b : binop
adamc@11 387 e1 : exp
adamc@11 388 IHe1 : forall (s : stack) (p : list instr),
adamc@11 389 progDenote (compile e1 ++ p) s = progDenote p (expDenote e1 :: s)
adamc@11 390 e2 : exp
adamc@11 391 IHe2 : forall (s : stack) (p : list instr),
adamc@11 392 progDenote (compile e2 ++ p) s = progDenote p (expDenote e2 :: s)
adamc@11 393 s : stack
adamc@11 394 p : list instr
adamc@11 395 ============================
adam@311 396 progDenote ((compile e2 ++ compile e1 ++ iBinop b :: nil) ++ p) s =
adamc@11 397 progDenote p (binopDenote b (expDenote e1) (expDenote e2) :: s)
adamc@206 398
adamc@11 399 ]]
adamc@11 400
adam@311 401 What we need is the associative law of list concatenation, which is available as a theorem [app_assoc_reverse] in the standard library.%\index{Vernacular commands!Check}% *)
adamc@11 402
adam@311 403 Check app_assoc.
adam@311 404
adamc@11 405 (** [[
adam@311 406 app_assoc_reverse
adamc@11 407 : forall (A : Type) (l m n : list A), (l ++ m) ++ n = l ++ m ++ n
adamc@206 408
adamc@11 409 ]]
adamc@11 410
adam@399 411 If we did not already know the name of the theorem, we could use the %\index{Vernacular commands!SearchRewrite}%[SearchRewrite] command to find it, based on a pattern that we would like to rewrite: *)
adam@277 412
adam@277 413 SearchRewrite ((_ ++ _) ++ _).
adam@277 414 (** [[
adam@311 415 app_assoc_reverse:
adam@311 416 forall (A : Type) (l m n : list A), (l ++ m) ++ n = l ++ m ++ n
adam@311 417 ]]
adam@311 418 %\vspace{-.25in}%
adam@311 419 [[
adam@311 420 app_assoc: forall (A : Type) (l m n : list A), l ++ m ++ n = (l ++ m) ++ n
adam@277 421
adam@277 422 ]]
adam@277 423
adam@311 424 We use [app_assoc_reverse] to perform a rewrite: %\index{tactics!rewrite}% *)
adamc@11 425
adam@311 426 rewrite app_assoc_reverse.
adamc@11 427
adamc@206 428 (** changing the conclusion to:
adamc@11 429
adamc@206 430 [[
adam@311 431 progDenote (compile e2 ++ (compile e1 ++ iBinop b :: nil) ++ p) s =
adamc@11 432 progDenote p (binopDenote b (expDenote e1) (expDenote e2) :: s)
adamc@206 433
adamc@11 434 ]]
adamc@11 435
adam@311 436 Now we can notice that the lefthand side of the equality matches the lefthand side of the second inductive hypothesis, so we can rewrite with that hypothesis, too.%\index{tactics!rewrite}% *)
adamc@11 437
adamc@4 438 rewrite IHe2.
adamc@11 439 (** [[
adam@311 440 progDenote ((compile e1 ++ iBinop b :: nil) ++ p) (expDenote e2 :: s) =
adamc@11 441 progDenote p (binopDenote b (expDenote e1) (expDenote e2) :: s)
adamc@206 442
adamc@11 443 ]]
adamc@11 444
adam@311 445 The same process lets us apply the remaining hypothesis.%\index{tactics!rewrite}% *)
adamc@11 446
adam@311 447 rewrite app_assoc_reverse.
adamc@4 448 rewrite IHe1.
adamc@11 449 (** [[
adam@311 450 progDenote ((iBinop b :: nil) ++ p) (expDenote e1 :: expDenote e2 :: s) =
adamc@11 451 progDenote p (binopDenote b (expDenote e1) (expDenote e2) :: s)
adamc@206 452
adamc@11 453 ]]
adamc@11 454
adam@311 455 Now we can apply a similar sequence of tactics to the one that ended the proof of the first case.%\index{tactics!unfold}\index{tactics!simpl}\index{tactics!fold}\index{tactics!reflexivity}%
adamc@11 456 *)
adamc@11 457
adamc@11 458 unfold progDenote at 1.
adamc@4 459 simpl.
adamc@11 460 fold progDenote.
adamc@4 461 reflexivity.
adamc@11 462
adam@311 463 (** And the proof is completed, as indicated by the message: *)
adamc@11 464
adam@399 465 (**
adam@399 466 <<
adam@399 467 Proof completed.
adam@399 468 >>
adam@399 469 *)
adamc@11 470
adam@311 471 (** And there lies our first proof. Already, even for simple theorems like this, the final proof script is unstructured and not very enlightening to readers. If we extend this approach to more serious theorems, we arrive at the unreadable proof scripts that are the favorite complaints of opponents of tactic-based proving. Fortunately, Coq has rich support for scripted automation, and we can take advantage of such a scripted tactic (defined elsewhere) to make short work of this lemma. We abort the old proof attempt and start again.%\index{Vernacular commands!Abort}%
adamc@11 472 *)
adamc@11 473
adamc@4 474 Abort.
adamc@2 475
adam@311 476 (** %\index{tactics!induction}\index{tactics!crush}% *)
adam@311 477
adamc@26 478 Lemma compile_correct' : forall e s p, progDenote (compile e ++ p) s =
adamc@4 479 progDenote p (expDenote e :: s).
adamc@4 480 induction e; crush.
adamc@4 481 Qed.
adamc@2 482
adam@328 483 (** We need only to state the basic inductive proof scheme and call a tactic that automates the tedious reasoning in between. In contrast to the period tactic terminator from our last proof, the %\index{tactics!semicolon}%semicolon tactic separator supports structured, compositional proofs. The tactic [t1; t2] has the effect of running [t1] and then running [t2] on each remaining subgoal. The semicolon is one of the most fundamental building blocks of effective proof automation. The period terminator is very useful for exploratory proving, where you need to see intermediate proof states, but final proofs of any serious complexity should have just one period, terminating a single compound tactic that probably uses semicolons.
adamc@11 484
adam@399 485 The [crush] tactic comes from the library associated with this book and is not part of the Coq standard library. The book's library contains a number of other tactics that are especially helpful in highly automated proofs.
adamc@210 486
adam@398 487 The %\index{Vernacular commands!Qed}%[Qed] command checks that the proof is finished and, if so, saves it. The tactic commands we have written above are an example of a _proof script_, or a series of Ltac programs; while [Qed] uses the result of the script to generate a _proof term_, a well-typed term of Gallina. To believe that a theorem is true, we only need to trust that the (relatively simple) checker for proof terms is correct; the use of proof scripts is immaterial. Part I of this book will introduce the principles behind encoding all proofs as terms of Gallina.
adam@311 488
adam@311 489 The proof of our main theorem is now easy. We prove it with four period-terminated tactics, though separating them with semicolons would work as well; the version here is easier to step through.%\index{tactics!intros}% *)
adamc@11 490
adamc@26 491 Theorem compile_correct : forall e, progDenote (compile e) nil = Some (expDenote e :: nil).
adamc@11 492 intros.
adamc@11 493 (** [[
adamc@11 494 e : exp
adamc@11 495 ============================
adamc@11 496 progDenote (compile e) nil = Some (expDenote e :: nil)
adamc@206 497
adamc@11 498 ]]
adamc@11 499
adamc@26 500 At this point, we want to massage the lefthand side to match the statement of [compile_correct']. A theorem from the standard library is useful: *)
adamc@11 501
adamc@11 502 Check app_nil_end.
adamc@11 503 (** [[
adamc@11 504 app_nil_end
adamc@11 505 : forall (A : Type) (l : list A), l = l ++ nil
adam@302 506 ]]
adam@311 507 %\index{tactics!rewrite}% *)
adamc@11 508
adamc@4 509 rewrite (app_nil_end (compile e)).
adamc@11 510
adam@417 511 (** This time, we explicitly specify the value of the variable [l] from the theorem statement, since multiple expressions of list type appear in the conclusion. The [rewrite] tactic might choose the wrong place to rewrite if we did not specify which we want.
adamc@11 512
adamc@11 513 [[
adamc@11 514 e : exp
adamc@11 515 ============================
adamc@11 516 progDenote (compile e ++ nil) nil = Some (expDenote e :: nil)
adamc@206 517
adamc@11 518 ]]
adamc@11 519
adam@311 520 Now we can apply the lemma.%\index{tactics!rewrite}% *)
adamc@11 521
adamc@26 522 rewrite compile_correct'.
adamc@11 523 (** [[
adamc@11 524 e : exp
adamc@11 525 ============================
adamc@11 526 progDenote nil (expDenote e :: nil) = Some (expDenote e :: nil)
adamc@206 527
adamc@11 528 ]]
adamc@11 529
adam@311 530 We are almost done. The lefthand and righthand sides can be seen to match by simple symbolic evaluation. That means we are in luck, because Coq identifies any pair of terms as equal whenever they normalize to the same result by symbolic evaluation. By the definition of [progDenote], that is the case here, but we do not need to worry about such details. A simple invocation of %\index{tactics!reflexivity}%[reflexivity] does the normalization and checks that the two results are syntactically equal.%\index{tactics!reflexivity}% *)
adamc@11 531
adamc@4 532 reflexivity.
adamc@4 533 Qed.
adamc@22 534 (* end thide *)
adamc@14 535
adam@311 536 (** This proof can be shortened and made automated, but we leave that as an exercise for the reader. *)
adam@311 537
adamc@14 538
adamc@20 539 (** * Typed Expressions *)
adamc@14 540
adamc@14 541 (** In this section, we will build on the initial example by adding additional expression forms that depend on static typing of terms for safety. *)
adamc@14 542
adamc@20 543 (** ** Source Language *)
adamc@14 544
adamc@15 545 (** We define a trivial language of types to classify our expressions: *)
adamc@15 546
adamc@14 547 Inductive type : Set := Nat | Bool.
adamc@14 548
adam@277 549 (** Like most programming languages, Coq uses case-sensitive variable names, so that our user-defined type [type] is distinct from the [Type] keyword that we have already seen appear in the statement of a polymorphic theorem (and that we will meet in more detail later), and our constructor names [Nat] and [Bool] are distinct from the types [nat] and [bool] in the standard library.
adam@277 550
adam@277 551 Now we define an expanded set of binary operators. *)
adamc@15 552
adamc@14 553 Inductive tbinop : type -> type -> type -> Set :=
adamc@14 554 | TPlus : tbinop Nat Nat Nat
adamc@14 555 | TTimes : tbinop Nat Nat Nat
adamc@14 556 | TEq : forall t, tbinop t t Bool
adamc@14 557 | TLt : tbinop Nat Nat Bool.
adamc@14 558
adam@398 559 (** The definition of [tbinop] is different from [binop] in an important way. Where we declared that [binop] has type [Set], here we declare that [tbinop] has type [type -> type -> type -> Set]. We define [tbinop] as an _indexed type family_. Indexed inductive types are at the heart of Coq's expressive power; almost everything else of interest is defined in terms of them.
adamc@15 560
adam@398 561 The inuitive explanation of [tbinop] is that a [tbinop t1 t2 t] is a binary operator whose operands should have types [t1] and [t2], and whose result has type [t]. For instance, constructor [TLt] (for less-than comparison of numbers) is assigned type [tbinop Nat Nat Bool], meaning the operator's arguments are naturals and its result is boolean. The type of [TEq] introduces a small bit of additional complication via polymorphism: we want to allow equality comparison of any two values of any type, as long as they have the _same_ type.
adam@312 562
adamc@15 563 ML and Haskell have indexed algebraic datatypes. For instance, their list types are indexed by the type of data that the list carries. However, compared to Coq, ML and Haskell 98 place two important restrictions on datatype definitions.
adamc@15 564
adam@417 565 First, the indices of the range of each data constructor must be type variables bound at the top level of the datatype definition. There is no way to do what we did here, where we, for instance, say that [TPlus] is a constructor building a [tbinop] whose indices are all fixed at [Nat]. %\index{generalized algebraic datatypes}\index{GADTs|see{generalized algebraic datatypes}}% _Generalized algebraic datatypes_ (GADTs)%~\cite{GADT}% are a popular feature in %\index{GHC Haskell}%GHC Haskell and other languages that removes this first restriction.
adamc@15 566
adam@417 567 The second restriction is not lifted by GADTs. In ML and Haskell, indices of types must be types and may not be _expressions_. In Coq, types may be indexed by arbitrary Gallina terms. Type indices can live in the same universe as programs, and we can compute with them just like regular programs. Haskell supports a hobbled form of computation in type indices based on %\index{Haskell}%multi-parameter type classes, and recent extensions like type functions bring Haskell programming even closer to %``%#"#real#"#%''% functional programming with types, but, without dependent typing, there must always be a gap between how one programs with types and how one programs normally.
adamc@15 568 *)
adamc@15 569
adam@399 570 (** We can define a similar type family for typed expressions, where a term of type [texp t] can be assigned object language type [t]. (It is conventional in the world of interactive theorem proving to call the language of the proof assistant the%\index{meta language}% _meta language_ and a language being formalized the%\index{object language}% _object language_.) *)
adamc@15 571
adamc@14 572 Inductive texp : type -> Set :=
adamc@14 573 | TNConst : nat -> texp Nat
adamc@14 574 | TBConst : bool -> texp Bool
adam@312 575 | TBinop : forall t1 t2 t, tbinop t1 t2 t -> texp t1 -> texp t2 -> texp t.
adamc@14 576
adamc@15 577 (** Thanks to our use of dependent types, every well-typed [texp] represents a well-typed source expression, by construction. This turns out to be very convenient for many things we might want to do with expressions. For instance, it is easy to adapt our interpreter approach to defining semantics. We start by defining a function mapping the types of our languages into Coq types: *)
adamc@15 578
adamc@14 579 Definition typeDenote (t : type) : Set :=
adamc@14 580 match t with
adamc@14 581 | Nat => nat
adamc@14 582 | Bool => bool
adamc@14 583 end.
adamc@14 584
adam@312 585 (** It can take a few moments to come to terms with the fact that [Set], the type of types of programs, is itself a first-class type, and that we can write functions that return [Set]s. Past that wrinkle, the definition of [typeDenote] is trivial, relying on the [nat] and [bool] types from the Coq standard library. We can interpret binary operators by relying on standard-library equality test functions [eqb] and [beq_nat] for booleans and naturals, respectively, along with a less-than test [leb]: *)
adamc@15 586
adamc@207 587 Definition tbinopDenote arg1 arg2 res (b : tbinop arg1 arg2 res)
adamc@207 588 : typeDenote arg1 -> typeDenote arg2 -> typeDenote res :=
adamc@207 589 match b with
adamc@207 590 | TPlus => plus
adamc@207 591 | TTimes => mult
adam@277 592 | TEq Nat => beq_nat
adam@277 593 | TEq Bool => eqb
adam@312 594 | TLt => leb
adamc@207 595 end.
adamc@207 596
adam@399 597 (** This function has just a few differences from the denotation functions we saw earlier. First, [tbinop] is an indexed type, so its indices become additional arguments to [tbinopDenote]. Second, we need to perform a genuine%\index{dependent pattern matching}% _dependent pattern match_, where the necessary _type_ of each case body depends on the _value_ that has been matched. At this early stage, we will not go into detail on the many subtle aspects of Gallina that support dependent pattern-matching, but the subject is central to Part II of the book.
adam@312 598
adamc@15 599 The same tricks suffice to define an expression denotation function in an unsurprising way:
adamc@15 600 *)
adamc@15 601
adamc@207 602 Fixpoint texpDenote t (e : texp t) : typeDenote t :=
adamc@207 603 match e with
adamc@14 604 | TNConst n => n
adamc@14 605 | TBConst b => b
adamc@14 606 | TBinop _ _ _ b e1 e2 => (tbinopDenote b) (texpDenote e1) (texpDenote e2)
adamc@14 607 end.
adamc@14 608
adamc@17 609 (** We can evaluate a few example programs to convince ourselves that this semantics is correct. *)
adamc@17 610
adamc@17 611 Eval simpl in texpDenote (TNConst 42).
adamc@207 612 (** [= 42 : typeDenote Nat] *)
adamc@207 613
adamc@17 614 Eval simpl in texpDenote (TBConst true).
adamc@207 615 (** [= true : typeDenote Bool] *)
adamc@207 616
adam@312 617 Eval simpl in texpDenote (TBinop TTimes (TBinop TPlus (TNConst 2) (TNConst 2))
adam@312 618 (TNConst 7)).
adamc@207 619 (** [= 28 : typeDenote Nat] *)
adamc@207 620
adam@312 621 Eval simpl in texpDenote (TBinop (TEq Nat) (TBinop TPlus (TNConst 2) (TNConst 2))
adam@312 622 (TNConst 7)).
adam@399 623 (** [= false : typeDenote Bool] *)
adamc@207 624
adam@312 625 Eval simpl in texpDenote (TBinop TLt (TBinop TPlus (TNConst 2) (TNConst 2))
adam@312 626 (TNConst 7)).
adamc@207 627 (** [= true : typeDenote Bool] *)
adamc@17 628
adamc@14 629
adamc@20 630 (** ** Target Language *)
adamc@14 631
adam@292 632 (** Now we want to define a suitable stack machine target for compilation. In the example of the untyped language, stack machine programs could encounter stack underflows and %``%#"#get stuck.#"#%''% This was unfortunate, since we had to deal with this complication even though we proved that our compiler never produced underflowing programs. We could have used dependent types to force all stack machine programs to be underflow-free.
adamc@18 633
adamc@18 634 For our new languages, besides underflow, we also have the problem of stack slots with naturals instead of bools or vice versa. This time, we will use indexed typed families to avoid the need to reason about potential failures.
adamc@18 635
adamc@18 636 We start by defining stack types, which classify sets of possible stacks. *)
adamc@18 637
adamc@14 638 Definition tstack := list type.
adamc@14 639
adamc@18 640 (** Any stack classified by a [tstack] must have exactly as many elements, and each stack element must have the type found in the same position of the stack type.
adamc@18 641
adamc@18 642 We can define instructions in terms of stack types, where every instruction's type tells us what initial stack type it expects and what final stack type it will produce. *)
adamc@18 643
adamc@14 644 Inductive tinstr : tstack -> tstack -> Set :=
adam@312 645 | TiNConst : forall s, nat -> tinstr s (Nat :: s)
adam@312 646 | TiBConst : forall s, bool -> tinstr s (Bool :: s)
adam@311 647 | TiBinop : forall arg1 arg2 res s,
adamc@14 648 tbinop arg1 arg2 res
adamc@14 649 -> tinstr (arg1 :: arg2 :: s) (res :: s).
adamc@14 650
adamc@18 651 (** Stack machine programs must be a similar inductive family, since, if we again used the [list] type family, we would not be able to guarantee that intermediate stack types match within a program. *)
adamc@18 652
adamc@14 653 Inductive tprog : tstack -> tstack -> Set :=
adamc@14 654 | TNil : forall s, tprog s s
adamc@14 655 | TCons : forall s1 s2 s3,
adamc@14 656 tinstr s1 s2
adamc@14 657 -> tprog s2 s3
adamc@14 658 -> tprog s1 s3.
adamc@14 659
adamc@18 660 (** Now, to define the semantics of our new target language, we need a representation for stacks at runtime. We will again take advantage of type information to define types of value stacks that, by construction, contain the right number and types of elements. *)
adamc@18 661
adamc@14 662 Fixpoint vstack (ts : tstack) : Set :=
adamc@14 663 match ts with
adamc@14 664 | nil => unit
adamc@14 665 | t :: ts' => typeDenote t * vstack ts'
adamc@14 666 end%type.
adamc@14 667
adam@312 668 (** This is another [Set]-valued function. This time it is recursive, which is perfectly valid, since [Set] is not treated specially in determining which functions may be written. We say that the value stack of an empty stack type is any value of type [unit], which has just a single value, [tt]. A nonempty stack type leads to a value stack that is a pair, whose first element has the proper type and whose second element follows the representation for the remainder of the stack type. We write [%]%\index{notation scopes}\coqdocvar{%#<tt>#type#</tt>#%}% as an instruction to Coq's extensible parser. In particular, this directive applies to the whole [match] expression, which we ask to be parsed as though it were a type, so that the operator [*] is interpreted as Cartesian product instead of, say, multiplication. (Note that this use of %\coqdocvar{%#<tt>#type#</tt>#%}% has no connection to the inductive type [type] that we have defined.)
adamc@18 669
adam@312 670 This idea of programming with types can take a while to internalize, but it enables a very simple definition of instruction denotation. Our definition is like what you might expect from a Lisp-like version of ML that ignored type information. Nonetheless, the fact that [tinstrDenote] passes the type-checker guarantees that our stack machine programs can never go wrong. We use a special form of [let] to destructure a multi-level tuple. *)
adamc@18 671
adamc@14 672 Definition tinstrDenote ts ts' (i : tinstr ts ts') : vstack ts -> vstack ts' :=
adamc@207 673 match i with
adam@312 674 | TiNConst _ n => fun s => (n, s)
adam@312 675 | TiBConst _ b => fun s => (b, s)
adam@311 676 | TiBinop _ _ _ _ b => fun s =>
adam@312 677 let '(arg1, (arg2, s')) := s in
adam@312 678 ((tbinopDenote b) arg1 arg2, s')
adamc@14 679 end.
adamc@14 680
adamc@18 681 (** Why do we choose to use an anonymous function to bind the initial stack in every case of the [match]? Consider this well-intentioned but invalid alternative version:
adamc@18 682 [[
adamc@18 683 Definition tinstrDenote ts ts' (i : tinstr ts ts') (s : vstack ts) : vstack ts' :=
adamc@207 684 match i with
adam@312 685 | TiNConst _ n => (n, s)
adam@312 686 | TiBConst _ b => (b, s)
adam@311 687 | TiBinop _ _ _ _ b =>
adam@312 688 let '(arg1, (arg2, s')) := s in
adam@312 689 ((tbinopDenote b) arg1 arg2, s')
adamc@18 690 end.
adamc@18 691
adamc@205 692 ]]
adamc@205 693
adamc@18 694 The Coq type-checker complains that:
adamc@18 695
adam@312 696 <<
adamc@18 697 The term "(n, s)" has type "(nat * vstack ts)%type"
adamc@207 698 while it is expected to have type "vstack ?119".
adam@312 699 >>
adamc@207 700
adam@312 701 This and other mysteries of Coq dependent typing we postpone until Part II of the book. The upshot of our later discussion is that it is often useful to push inside of [match] branches those function parameters whose types depend on the type of the value being matched. Our later, more complete treatement of Gallina's typing rules will explain why this helps.
adamc@18 702 *)
adamc@18 703
adamc@18 704 (** We finish the semantics with a straightforward definition of program denotation. *)
adamc@18 705
adamc@207 706 Fixpoint tprogDenote ts ts' (p : tprog ts ts') : vstack ts -> vstack ts' :=
adamc@207 707 match p with
adamc@14 708 | TNil _ => fun s => s
adamc@14 709 | TCons _ _ _ i p' => fun s => tprogDenote p' (tinstrDenote i s)
adamc@14 710 end.
adamc@14 711
adamc@14 712
adamc@14 713 (** ** Translation *)
adamc@14 714
adamc@19 715 (** To define our compilation, it is useful to have an auxiliary function for concatenating two stack machine programs. *)
adamc@19 716
adamc@207 717 Fixpoint tconcat ts ts' ts'' (p : tprog ts ts') : tprog ts' ts'' -> tprog ts ts'' :=
adamc@207 718 match p with
adamc@14 719 | TNil _ => fun p' => p'
adamc@14 720 | TCons _ _ _ i p1 => fun p' => TCons i (tconcat p1 p')
adamc@14 721 end.
adamc@14 722
adamc@19 723 (** With that function in place, the compilation is defined very similarly to how it was before, modulo the use of dependent typing. *)
adamc@19 724
adamc@207 725 Fixpoint tcompile t (e : texp t) (ts : tstack) : tprog ts (t :: ts) :=
adamc@207 726 match e with
adam@312 727 | TNConst n => TCons (TiNConst _ n) (TNil _)
adam@312 728 | TBConst b => TCons (TiBConst _ b) (TNil _)
adamc@14 729 | TBinop _ _ _ b e1 e2 => tconcat (tcompile e2 _)
adam@311 730 (tconcat (tcompile e1 _) (TCons (TiBinop _ b) (TNil _)))
adamc@14 731 end.
adamc@14 732
adam@398 733 (** One interesting feature of the definition is the underscores appearing to the right of [=>] arrows. Haskell and ML programmers are quite familiar with compilers that infer type parameters to polymorphic values. In Coq, it is possible to go even further and ask the system to infer arbitrary terms, by writing underscores in place of specific values. You may have noticed that we have been calling functions without specifying all of their arguments. For instance, the recursive calls here to [tcompile] omit the [t] argument. Coq's _implicit argument_ mechanism automatically inserts underscores for arguments that it will probably be able to infer. Inference of such values is far from complete, though; generally, it only works in cases similar to those encountered with polymorphic type instantiation in Haskell and ML.
adamc@19 734
adamc@19 735 The underscores here are being filled in with stack types. That is, the Coq type inferencer is, in a sense, inferring something about the flow of control in the translated programs. We can take a look at exactly which values are filled in: *)
adamc@19 736
adamc@14 737 Print tcompile.
adamc@19 738 (** [[
adamc@19 739 tcompile =
adamc@19 740 fix tcompile (t : type) (e : texp t) (ts : tstack) {struct e} :
adamc@19 741 tprog ts (t :: ts) :=
adamc@19 742 match e in (texp t0) return (tprog ts (t0 :: ts)) with
adam@312 743 | TNConst n => TCons (TiNConst ts n) (TNil (Nat :: ts))
adam@312 744 | TBConst b => TCons (TiBConst ts b) (TNil (Bool :: ts))
adamc@19 745 | TBinop arg1 arg2 res b e1 e2 =>
adamc@19 746 tconcat (tcompile arg2 e2 ts)
adamc@19 747 (tconcat (tcompile arg1 e1 (arg2 :: ts))
adam@311 748 (TCons (TiBinop ts b) (TNil (res :: ts))))
adamc@19 749 end
adamc@19 750 : forall t : type, texp t -> forall ts : tstack, tprog ts (t :: ts)
adam@302 751 ]]
adam@302 752 *)
adamc@19 753
adamc@19 754
adamc@19 755 (** We can check that the compiler generates programs that behave appropriately on our sample programs from above: *)
adamc@19 756
adamc@19 757 Eval simpl in tprogDenote (tcompile (TNConst 42) nil) tt.
adam@399 758 (** [= (42, tt) : vstack (Nat :: nil)] *)
adamc@207 759
adamc@19 760 Eval simpl in tprogDenote (tcompile (TBConst true) nil) tt.
adam@399 761 (** [= (true, tt) : vstack (Bool :: nil)] *)
adamc@207 762
adam@312 763 Eval simpl in tprogDenote (tcompile (TBinop TTimes (TBinop TPlus (TNConst 2)
adam@312 764 (TNConst 2)) (TNConst 7)) nil) tt.
adam@399 765 (** [= (28, tt) : vstack (Nat :: nil)] *)
adamc@207 766
adam@312 767 Eval simpl in tprogDenote (tcompile (TBinop (TEq Nat) (TBinop TPlus (TNConst 2)
adam@312 768 (TNConst 2)) (TNConst 7)) nil) tt.
adam@399 769 (** [= (false, tt) : vstack (Bool :: nil)] *)
adamc@207 770
adam@312 771 Eval simpl in tprogDenote (tcompile (TBinop TLt (TBinop TPlus (TNConst 2) (TNConst 2))
adam@312 772 (TNConst 7)) nil) tt.
adam@399 773 (** [= (true, tt) : vstack (Bool :: nil)] *)
adamc@19 774
adamc@14 775
adamc@20 776 (** ** Translation Correctness *)
adamc@20 777
adamc@20 778 (** We can state a correctness theorem similar to the last one. *)
adamc@20 779
adamc@207 780 Theorem tcompile_correct : forall t (e : texp t),
adamc@207 781 tprogDenote (tcompile e nil) tt = (texpDenote e, tt).
adamc@20 782 (* begin hide *)
adamc@20 783 Abort.
adamc@20 784 (* end hide *)
adamc@22 785 (* begin thide *)
adamc@20 786
adam@312 787 (** Again, we need to strengthen the theorem statement so that the induction will go through. This time, to provide an excuse to demonstrate different tactics, I will develop an alternative approach to this kind of proof, stating the key lemma as: *)
adamc@14 788
adamc@207 789 Lemma tcompile_correct' : forall t (e : texp t) ts (s : vstack ts),
adamc@207 790 tprogDenote (tcompile e ts) s = (texpDenote e, s).
adamc@20 791
adam@394 792 (** While lemma [compile_correct'] quantified over a program that is the %``%#"#continuation#"#%''~\cite{continuations}% for the expression we are considering, here we avoid drawing in any extra syntactic elements. In addition to the source expression and its type, we also quantify over an initial stack type and a stack compatible with it. Running the compilation of the program starting from that stack, we should arrive at a stack that differs only in having the program's denotation pushed onto it.
adamc@20 793
adamc@20 794 Let us try to prove this theorem in the same way that we settled on in the last section. *)
adamc@20 795
adamc@14 796 induction e; crush.
adamc@20 797
adamc@20 798 (** We are left with this unproved conclusion:
adamc@20 799
adamc@20 800 [[
adamc@20 801 tprogDenote
adamc@20 802 (tconcat (tcompile e2 ts)
adamc@20 803 (tconcat (tcompile e1 (arg2 :: ts))
adam@311 804 (TCons (TiBinop ts t) (TNil (res :: ts))))) s =
adamc@20 805 (tbinopDenote t (texpDenote e1) (texpDenote e2), s)
adamc@207 806
adamc@20 807 ]]
adamc@20 808
adam@312 809 We need an analogue to the [app_assoc_reverse] theorem that we used to rewrite the goal in the last section. We can abort this proof and prove such a lemma about [tconcat].
adamc@20 810 *)
adamc@207 811
adamc@14 812 Abort.
adamc@14 813
adamc@26 814 Lemma tconcat_correct : forall ts ts' ts'' (p : tprog ts ts') (p' : tprog ts' ts'')
adamc@14 815 (s : vstack ts),
adamc@14 816 tprogDenote (tconcat p p') s
adamc@14 817 = tprogDenote p' (tprogDenote p s).
adamc@14 818 induction p; crush.
adamc@14 819 Qed.
adamc@14 820
adamc@20 821 (** This one goes through completely automatically.
adamc@20 822
adam@316 823 Some code behind the scenes registers [app_assoc_reverse] for use by [crush]. We must register [tconcat_correct] similarly to get the same effect:%\index{Vernacular commands!Hint Rewrite}% *)
adamc@20 824
adam@375 825 Hint Rewrite tconcat_correct.
adamc@14 826
adam@398 827 (** Here we meet the pervasive concept of a _hint_. Many proofs can be found through exhaustive enumerations of combinations of possible proof steps; hints provide the set of steps to consider. The tactic [crush] is applying such brute force search for us silently, and it will consider more possibilities as we add more hints. This particular hint asks that the lemma be used for left-to-right rewriting, and we ask for the hint to be added to the hint database called [cpdt], which is the database used by [crush]. In general, segragating hints into different databases is helpful to control the performance of proof search, in cases where domain knowledge allows us to narrow the set of proof steps to be considered in brute force search. Part III of this book considers such pragmatic aspects of proof search in much more detail.
adam@312 828
adam@312 829 Now we are ready to return to [tcompile_correct'], proving it automatically this time. *)
adamc@20 830
adamc@207 831 Lemma tcompile_correct' : forall t (e : texp t) ts (s : vstack ts),
adamc@207 832 tprogDenote (tcompile e ts) s = (texpDenote e, s).
adamc@14 833 induction e; crush.
adamc@14 834 Qed.
adamc@14 835
adamc@20 836 (** We can register this main lemma as another hint, allowing us to prove the final theorem trivially. *)
adamc@20 837
adam@375 838 Hint Rewrite tcompile_correct'.
adamc@14 839
adamc@207 840 Theorem tcompile_correct : forall t (e : texp t),
adamc@207 841 tprogDenote (tcompile e nil) tt = (texpDenote e, tt).
adamc@14 842 crush.
adamc@14 843 Qed.
adamc@22 844 (* end thide *)
adam@312 845
adam@399 846 (** It is probably worth emphasizing that we are doing more than building mathematical models. Our compilers are functional programs that can be executed efficiently. One strategy for doing so is based on%\index{program extraction}% _program extraction_, which generates OCaml code from Coq developments. For instance, we run a command to output the OCaml version of [tcompile]:%\index{Vernacular commands!Extraction}% *)
adam@312 847
adam@312 848 Extraction tcompile.
adam@312 849
adam@312 850 (** <<
adam@312 851 let rec tcompile t e ts =
adam@312 852 match e with
adam@312 853 | TNConst n ->
adam@312 854 TCons (ts, (Cons (Nat, ts)), (Cons (Nat, ts)), (TiNConst (ts, n)), (TNil
adam@312 855 (Cons (Nat, ts))))
adam@312 856 | TBConst b ->
adam@312 857 TCons (ts, (Cons (Bool, ts)), (Cons (Bool, ts)), (TiBConst (ts, b)),
adam@312 858 (TNil (Cons (Bool, ts))))
adam@312 859 | TBinop (t1, t2, t0, b, e1, e2) ->
adam@312 860 tconcat ts (Cons (t2, ts)) (Cons (t0, ts)) (tcompile t2 e2 ts)
adam@312 861 (tconcat (Cons (t2, ts)) (Cons (t1, (Cons (t2, ts)))) (Cons (t0, ts))
adam@312 862 (tcompile t1 e1 (Cons (t2, ts))) (TCons ((Cons (t1, (Cons (t2,
adam@312 863 ts)))), (Cons (t0, ts)), (Cons (t0, ts)), (TiBinop (t1, t2, t0, ts,
adam@312 864 b)), (TNil (Cons (t0, ts))))))
adam@312 865 >>
adam@312 866
adam@312 867 We can compile this code with the usual OCaml compiler and obtain an executable program with halfway decent performance.
adam@312 868
adam@312 869 This chapter has been a whirlwind tour through two examples of the style of Coq development that I advocate. Parts II and III of the book focus on the key elements of that style, namely dependent types and scripted proof automation, respectively. Before we get there, we will spend some time in Part I on more standard foundational material. Part I may still be of interest to seasoned Coq hackers, since I follow the highly automated proof style even at that early stage. *)