[Haskell-beginners] parse String -> Expression

[Felipe Lessa]

On Sun, Nov 08, 2009 at 05:53:48PM +0000, John Moore wrote:
> Hi,
>     I'm trying to find a way to parseRPN (Reverse Polish Numbers) to
> expressions rather than to just numbers. e.g. I want the answer to be in the
> form
>
> (Multiply (Val 2) (Val 3)) rather than just the answer.

You don't need to code all the parser by hand.  You can use, for
example, Parsec parsers.

Spoilers ahead!!!






If your data type is

data Expr a = Multiply (Expr a) (Expr a)
            | Val a

then you may write something like

expression :: Parser a -> Parser (Expr a)
expression valParser = spaces >> (mult <|> val)
  where
    expr = expression valParser
    mult = char "*" >> (Multiply <$> expr <*> expr)
    val  = Val <$> valParser

using Parsec for a concrete example.

HTH,

--
Felipe.