ImpParser: Lexing and Parsing in Coq

The development of the Imp language in Imp.v completely ignores issues of concrete syntax -- how an ascii string that a programmer might write gets translated into abstract syntax trees defined by the datatypes aexp, bexp, and com. In this chapter, we illustrate how the rest of the story can be filled in by building a simple lexical analyzer and parser using Coq's functional programming facilities.

It is not important to understand all the details here (and accordingly, the explanations are fairly terse and there are no exercises). The main point is simply to demonstrate that it can be done. You are invited to look through the code -- most of it is not very complicated, though the parser relies on some monadic programming idioms that may require a little work to make out -- but most readers will probably want to just skim down to the Examples section at the very end to get the punchline.

Internals

Lexical Analysis

Definition isWhite (c : ascii) : bool :=
  let n := nat_of_ascii c in
  orb (orb (n =? 32)   (* space *)
           (n =? 9))   (* tab *)
      (orb (n =? 10)   (* linefeed *)
           (n =? 13)). (* Carriage return. *)

Notation "x '<=?' y" := (x <=? y)
  (at level 70, no associativity) : nat_scope.

Definition isLowerAlpha (c : ascii) : bool :=
  let n := nat_of_ascii c in
    andb (97 <=? n) (n <=? 122).

Definition isAlpha (c : ascii) : bool :=
  let n := nat_of_ascii c in
    orb (andb (65 <=? n) (n <=? 90))
        (andb (97 <=? n) (n <=? 122)).

Definition isDigit (c : ascii) : bool :=
  let n := nat_of_ascii c in
     andb (48 <=? n) (n <=? 57).

Inductive chartype := white | alpha | digit | other.

Definition classifyChar (c : ascii) : chartype :=
  if isWhite c then
    white
  else if isAlpha c then
    alpha
  else if isDigit c then
    digit
  else
    other.

Fixpoint list_of_string (s : string) : list ascii :=
  match s with
  | EmptyString => []
  | String c s => c :: (list_of_string s)
  end.

Fixpoint string_of_list (xs : list ascii) : string :=
  fold_right String EmptyString xs.

Definition token := string.

Fixpoint tokenize_helper (cls : chartype) (acc xs : list ascii)
                       : list (list ascii) :=
  let tk := match acc with [] => [] | _::_ => [rev acc] end in
  match xs with
  | [] => tk
  | (x::xs') =>
    match cls, classifyChar x, x with
    | _, _, "("      =>
      tk ++ ["("]::(tokenize_helper other [] xs')
    | _, _, ")"      =>
      tk ++ [")"]::(tokenize_helper other [] xs')
    | _, white, _    =>
      tk ++ (tokenize_helper white [] xs')
    | alpha,alpha,x  =>
      tokenize_helper alpha (x::acc) xs'
    | digit,digit,x  =>
      tokenize_helper digit (x::acc) xs'
    | other,other,x  =>
      tokenize_helper other (x::acc) xs'
    | _,tp,x         =>
      tk ++ (tokenize_helper tp [x] xs')
    end
  end %char.

Definition tokenize (s : string) : list string :=
  map string_of_list (tokenize_helper white [] (list_of_string s)).

Example tokenize_ex1 :
    tokenize "abc12=3  223*(3+(a+c))" %string
  = ["abc"; "12"; "="; "3"; "223";
       "*"; "("; "3"; "+"; "(";
       "a"; "+"; "c"; ")"; ")"]%string.
Proof. reflexivity. Qed.

(* ================================================================= *)

Parsing

Options With Errors

An option type with error messages:

Some syntactic sugar to make writing nested match-expressions on optionE more convenient.

Generic Combinators for Building Parsers

A (step-indexed) parser that expects zero or more ps:

A parser that expects a given token, followed by p:

A parser that expects a particular token:

A Recursive-Descent Parser for Imp

Identifiers:

Numbers:

Parse arithmetic expressions

Fixpoint parsePrimaryExp (steps:nat)
                         (xs : list token)
                       : optionE (aexp * list token) :=
  match steps with
  | 0 => NoneE "Too many recursive calls"
  | S steps' =>
      TRY ' (i, rest) <- parseIdentifier xs ;;
          SomeE (AId i, rest)
      OR
      TRY ' (n, rest) <- parseNumber xs ;;
          SomeE (ANum n, rest)
      OR
      ' (e, rest) <- firstExpect "(" (parseSumExp steps') xs ;;
      ' (u, rest') <- expect ")" rest ;;
      SomeE (e,rest')
  end

with parseProductExp (steps:nat)
                     (xs : list token) :=
  match steps with
  | 0 => NoneE "Too many recursive calls"
  | S steps' =>
    ' (e, rest) <- parsePrimaryExp steps' xs ;;
    ' (es, rest') <- many (firstExpect "*" (parsePrimaryExp steps'))
                          steps' rest ;;
    SomeE (fold_left AMult es e, rest')
  end

with parseSumExp (steps:nat) (xs : list token)  :=
  match steps with
  | 0 => NoneE "Too many recursive calls"
  | S steps' =>
    ' (e, rest) <- parseProductExp steps' xs ;;
    ' (es, rest') <-
        many (fun xs =>
                TRY ' (e,rest') <-
                    firstExpect "+"
                                (parseProductExp steps') xs ;;
                    SomeE ( (true, e), rest')
                OR
                ' (e, rest') <-
                    firstExpect "-"
                                (parseProductExp steps') xs ;;
                SomeE ( (false, e), rest'))
        steps' rest ;;
      SomeE (fold_left (fun e0 term =>
                          match term with
                          | (true,  e) => APlus e0 e
                          | (false, e) => AMinus e0 e
                          end)
                       es e,
             rest')
  end.

Definition parseAExp := parseSumExp.

Parsing boolean expressions:

Fixpoint parseAtomicExp (steps:nat)
                        (xs : list token)  :=
match steps with
  | 0 => NoneE "Too many recursive calls"
  | S steps' =>
     TRY ' (u,rest) <- expect "true" xs ;;
         SomeE (BTrue,rest)
     OR
     TRY ' (u,rest) <- expect "false" xs ;;
         SomeE (BFalse,rest)
     OR
     TRY ' (e,rest) <- firstExpect "~"
                                   (parseAtomicExp steps')
                                   xs ;;
         SomeE (BNot e, rest)
     OR
     TRY ' (e,rest) <- firstExpect "("
                                   (parseConjunctionExp steps')
                                   xs ;;
         ' (u,rest') <- expect ")" rest ;;
         SomeE (e, rest')
     OR
     ' (e, rest) <- parseProductExp steps' xs ;;
     TRY ' (e', rest') <- firstExpect "="
                                  (parseAExp steps') rest ;;
         SomeE (BEq e e', rest')
     OR
     TRY ' (e', rest') <- firstExpect "<="
                                      (parseAExp steps') rest ;;
         SomeE (BLe e e', rest')
     OR
     NoneE "Expected '=' or '<=' after arithmetic expression"
end

with parseConjunctionExp (steps:nat)
                         (xs : list token) :=
  match steps with
  | 0 => NoneE "Too many recursive calls"
  | S steps' =>
    ' (e, rest) <- parseAtomicExp steps' xs ;;
    ' (es, rest') <- many (firstExpect "&&"
               (parseAtomicExp steps'))
            steps' rest ;;
    SomeE (fold_left BAnd es e, rest')
  end.

Definition parseBExp := parseConjunctionExp.

Check parseConjunctionExp.

Definition testParsing {X : Type}
           (p : nat ->
                list token ->
                optionE (X * list token))
           (s : string) :=
  let t := tokenize s in
  p 100 t.

(*
Eval compute in
  testParsing parseProductExp "x.y.(x.x).x".

Eval compute in
  testParsing parseConjunctionExp "~(x=x&&x*x<=(x*x)*x)&&x=x".
*)

Parsing commands:

Fixpoint parseSimpleCommand (steps:nat)
                            (xs : list token) :=
  match steps with
  | 0 => NoneE "Too many recursive calls"
  | S steps' =>
    TRY ' (u, rest) <- expect "SKIP" xs ;;
        SomeE (SKIP%imp, rest)
    OR
    TRY ' (e,rest) <-
            firstExpect "TEST"
                        (parseBExp steps') xs ;;
        ' (c,rest') <-
            firstExpect "THEN"
                        (parseSequencedCommand steps') rest ;;
        ' (c',rest'') <-
            firstExpect "ELSE"
                        (parseSequencedCommand steps') rest' ;;
        ' (tt,rest''') <-
            expect "END" rest'' ;;
       SomeE(TEST e THEN c ELSE c' FI%imp, rest''')
    OR
    TRY ' (e,rest) <-
            firstExpect "WHILE"
                        (parseBExp steps') xs ;;
        ' (c,rest') <-
            firstExpect "DO"
                        (parseSequencedCommand steps') rest ;;
        ' (u,rest'') <-
            expect "END" rest' ;;
        SomeE(WHILE e DO c END%imp, rest'')
    OR
    TRY ' (i, rest) <- parseIdentifier xs ;;
        ' (e, rest') <- firstExpect "::=" (parseAExp steps') rest ;;
        SomeE ((i ::= e)%imp, rest')
    OR
        NoneE "Expecting a command"
end

with parseSequencedCommand (steps:nat)
                           (xs : list token) :=
  match steps with
  | 0 => NoneE "Too many recursive calls"
  | S steps' =>
    ' (c, rest) <- parseSimpleCommand steps' xs ;;
    TRY ' (c', rest') <-
            firstExpect ";;"
                        (parseSequencedCommand steps') rest ;;
        SomeE ((c ;; c')%imp, rest')
    OR
    SomeE (c, rest)
  end.

Definition bignumber := 1000.

Definition parse (str : string) : optionE com :=
  let tokens := tokenize str in
  match parseSequencedCommand bignumber tokens with
  | SomeE (c, []) => SomeE c
  | SomeE (_, t::_) => NoneE ("Trailing tokens remaining: " ++ t)
  | NoneE err => NoneE err
  end.

(*  *)