[Haskell-cafe] Newbie question on Haskell type

[Huong Nguyen]

Thanks all of your for your time and your interesting examples. Now I can
see that my problem is parsing a String. I am new in Haskell, so, I start to
study parsing and how to create a parser from beginning.

I start with an example from the book as follows:
%The parser item fails if the input is empty and consumes the first
character otherwise.

\begin{code}
newtype Parser a = Parser(String -> [(a, String)])

item::Parser Char
item = Parser(\cs -> case cs of
"" -> []
(c:cs) -> [(c,cs)])

parse :: Parser a -> String -> [(a, String)]
parse p cs = p cs
\end{code}

and I compile, the error displays. I do not know how to fix it. Please help
me.

$ghci parser.lhs
parser.lhs:10:13:
Couldn't match `Parser a' against `t -> t1'
Expected type: Parser a
Inferred type: t -> t1
Probable cause: `p' is applied to too many arguments in the call (p cs)
In the definition of `parse': parse p cs = p cs
Failed, modules loaded: none.



On 10/14/05, Ralf Hinze <ralf at informatik.uni-bonn.de> wrote:
>
> Hi Huong,
>
> attached you find a small program for parsing values of various (data)
> types. It uses a generalized algebraic data type for representing types
> and a universal data type for representing values. The parser itself is
> rather simple-minded: it builds on Haskell's "ReadS" type.
>
> I don't know whether this is what you are after, but it was fun writing.
> There are many opportunities for improvement: one could use a decent
> combinator library for parsing; a type of dynamic values instead of a
> universal type etc.
>
> Here are some example calls:
>
> Main> parseAny "4711"
> [(ValInt 4711,"")]
> Main> parseAny "\"4711\""
> [(ValString "4711","")]
> Main> parseAny "[4711, 0]"
> [(ValList [ValInt 4711,ValInt 0],"")]
> Main> parseAny "[4711, 'a']"
> [(ValList [ValInt 4711,ValChar 'a'],"")]
> Main> parseAny "[\"hello world\"]"
> [(ValList [ValString "hello world"],"")]
>
> Note that "parseAny" even parses heterogenous lists.
>
> Cheers, Ralf
>
> ---
>
> > {-# OPTIONS -fglasgow-exts #-}
>
> > data Type :: * -> * where
> > Char :: Type Char
> > Int :: Type Int
> > List :: Type a -> Type [a]
> > Value :: Type Value
>
> > string :: Type String
> > string = List Char
>
> > parse :: Type t -> ReadS t
> > parse (Char) = reads
> > parse (Int) = reads
> > parse (List Char) = reads
> > parse (List a) = parseList (parse (a))
> > parse (Value) = parseAny
>
> > data Value
> > = ValChar Char
> > | ValInt Int
> > | ValString String
> > | ValList [Value]
> > deriving (Show)
>
> > parseAny
> > = ValChar <$> parse Char
> > <+> ValInt <$> parse Int
> > <+> ValString <$> parse string
> > <+> ValList <$> parse (List Value)
>
> Helper functions.
>
> > parseList parsea
> > = readParen False (\ s -> [ xs | ("[", t) <- lex s, xs <- parsel t ])
> > where parsel s = [ ([], t) | ("]", t) <- lex s ]
> > ++ [ (x : xs, u) | (x, t) <- parsea s,
> > (xs, u) <- parsel' t ]
> > parsel' s = [ ([], t) | ("]", t) <- lex s ]
> > ++ [ (x : xs, v) | (",", t) <- lex s,
> > (x, u) <- parsea t,
> > (xs, v) <- parsel' u]
>
> > infix 8 <$>
> > infixr 6 <+>
> > (f <$> p) s = [ (f a, t) | (a, t) <- p s ]
> > (p <+> q) s = p s ++ q s
>
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