「SF-LC」13 ImpParser

the parser relies on some “monadic” programming idioms

basically, parser combinator (But 非常麻烦 in Coq)

Lex

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.
  

Definition token := string.

Syntax

带 error msg 的 option:

Inductive optionE (X:Type) : Type :=
  | SomeE (x : X)
  | NoneE (s : string).       (** w/ error msg **)

Arguments SomeE {X}.
Arguments NoneE {X}.

Monadic:

Notation "' p <- e1 ;; e2"
   := (match e1 with
       | SomeE p ⇒ e2
       | NoneE err ⇒ NoneE err
       end)
   (right associativity, p pattern, at level 60, e1 at next level).

Notation "'TRY' ' p <- e1 ;; e2 'OR' e3"
   := (match e1 with
       | SomeE p ⇒ e2
       | NoneE _ ⇒ e3
       end)
   (right associativity, p pattern,
    at level 60, e1 at next level, e2 at next level).

Definition parser (T : Type) :=
  list token → optionE (T * list token).

newtype Parser a = Parser (String -> [(a,String)])

instance Monad Parser where
   -- (>>=) :: Parser a -> (a -> Parser b) -> Parser b
   p >>= f = P (\inp -> case parse p inp of
                           []        -> []
                           [(v,out)] -> parse (f v) out)

combinator many

Coq vs. Haskell

1. explicit recursion depth, .e. step-indexed
2. explicit exception optionE (in Haskell, it’s hidden behind the Parser Monad as [])
3. explicit string state xs (in Haskell, it’s hidden behind the Parser Monad as String -> String)
4. explicit accepted token (in Haskell, it’s hidden behind the Parser Monad as a, argument)

Fixpoint many_helper {T} (p : parser T) acc steps xs :=
  match steps, p xs with
  | 0, _ ⇒
      NoneE "Too many recursive calls"
  | _, NoneE _ ⇒
      SomeE ((rev acc), xs)
  | S steps', SomeE (t, xs') ⇒
      many_helper p (t :: acc) steps' xs'
  end.

Fixpoint many {T} (p : parser T) (steps : nat) : parser (list T) :=
  many_helper p [] steps.

manyL :: Parser a -> Parser [a]
manyL p = many1L p <++ return []   -- left biased OR

many1L :: Parser a -> Parser [a]
many1L p = (:) <$> p <*> manyL p
-- or
many1L p = do x <- p
              xs <- manyL p
              return (x : xs)

ident

Definition parseIdentifier (xs : list token) : optionE (string * list token) :=
  match xs with
  | [] ⇒ NoneE "Expected identifier"
  | x::xs' ⇒ if forallb isLowerAlpha (list_of_string x)
             then SomeE (x, xs')
             else NoneE ("Illegal identifier:'" ++ x ++ "'")
  end.

ident :: Parser String
ident = do x  <- lower
           xs <- many alphanum
           return (x:xs)