[Haskell-beginners] prefix instead of infix

[Daniel Fischer]

Am Sonntag 10 Januar 2010 20:33:40 schrieb John Moore:
> Hi,
>    Can anyone explain how to turn the code from infix to Prefix. I want
> to include a let statement below. I not sure of how prefix  works.

Can you elaborate? I don't understand what it is you want to do.
Do you want to parse expressions in prefix notation (aka Polish notation), 
such as +(3,4) (or, more Lispy, (+ 3 4))?
That'd be simple, but you wouldn't use buldExpressionParser for that (in 
Polish notation, neither associativity nor precedence have a meaning).
Or do you want to add a parser for let-expressions to expr?

>
> expr :: Parser Expr
> expr = buildExpressionParser table factor
>    <?> "expression"
> table :: [[Operator Char st Expr]]
> table = [[op "*" Mul AssocLeft, op "/" Div AssocLeft]
>        ,[op "+" Add AssocLeft, op "-" Sub AssocLeft]
>        ]
>  where
>    op s f assoc
>       = Infix (do{ string s; return f}) assoc
> factor :: Parser Expr
> factor = do{ char '('
>           ; x <- expr
>           ; char ')'
>           ; return x
>           }
>      <|> number
>      <?> "simple expression"
> number :: Parser Expr
> number = do{ ds <- many1 alphaNum
>           ; return (Val $ read ds)
>           }
>
> John