On 6/4/07, Juozas Gaigalas
Hello,
I am a somewhat experienced programmer and a complete Haskell newbie, so I hope this is the correct ML for my question.
I have decided to learn Haskell and started with Graham Hutton's book. Everything was going nicely until section 8.4, on sequencing functional parsers. I am trying to write an elementary parser that produces the 1st and 3d elements from a string. I am using the code from the book.
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type Parser a = String -> [(a, String)]
return :: a -> Parser a return v = \inp -> [(v, inp)]
failure :: Parser a failure = \inp -> []
item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x, xs)]
parse :: Parser a -> String -> [(a, String)] parse p inp = p inp
(>>=) :: Parser a -> (a -> Parser b) -> Parser b p >>= f = \inp -> case parse p inp of [] -> [] [(v, out)] -> parse (f v) out
p :: Parser (Char, Char) p = do x <- item item y <- item return (x, y) -- LINE 34 --------------------
BUT, when I try to :load parse.hs from hugs I get the following error:
ERROR "parse.hs":34 - Last generator in do {...} must be an expression
Although the error is not really friendly (your "supposedly" last generator in do is an expression) the problem probably is indentation: Try like this
p = do x <- item item y <- item return (x, y) -- LINE 34
Hugs is probably complaining because it identifies "x <- item" (which is not a simple expression) as the last element of do.