On Fri, Feb 23, 2007 at 11:18:46PM -0500, David Cabana wrote:
I have been trying to work through Graham Hutton's "Programming in Haskell", but have hit something of a snag in chapter 8.4. Hutton presents some sample code which I am trying to run, with no luck so far. Here is the code as I constructed it by gathering snippets presented across three pages. <code> module Main where
-- as per Hutton page 75 type Parser a = String -> [(a, String)]
-- page 75 return :: a -> Parser a return v = \inp -> [(v,inp)]
This return conflicts with the one in Prelude, and (while similar) they are not interchangable.
-- page 76 failure :: Parser a failure = \inp -> [ ]
This is analogous to Prelude.fail. Fortunately Hutton didn't call it that :)
item :: Parser Char item = \inp -> case inp of [ ] -> [ ] (x:xs)-> [(x,xs)]
Looks reasonable
parse :: Parser a -> String -> [(a,String)] parse p inp = p inp
Same here
-- page 77 p :: Parser (Char, Char) p = do x <- item item y <- item return (x,y) </code>
Bad! Due to the Layout Rule that is parsed as a single long statement... I'm quite suprised you didn't get a parse error. It needs to be: p :: Parser (Char, Char) p = do x <- item item y <- item return (x,y) But, this still won't work. essentially the 'do' uses Prelude.return, Prelude.(>>), and Prelude.(>>=), which work on defined Monads; but your parser type is not properly declared as a monad. (and cannot be, because it is a type synonym.) You could define: (>>) :: Parser x -> Parser y -> Parser y (p1 >> p2) l = [ (s,rs2) | (f,rs1) <- p1 l , (s,rs2) <- p2 rs1 ] (>>=) :: Parser x -> (x -> Parser y) -> Parser y (p1 >>= fn) l = [ (s,rs2) | (f,rs1) <- p1 l , (s,rs2) <- fn f rs1 ] then use those (do-notation ignores scope so it must be desugared): p :: Parser (Char, Char) p = item Main.>>= \x -> item Main.>> item Main.>>= \y -> Main.return (x,y) This should work. Famous last words I know :)
When I tried to load this code, I got this error message:
Ambiguous occurrence `return' It could refer to either `return', defined at /Users/joe/haskell/ parse2.hs:8:0 or `return', imported from Prelude
OK. My reasoning was that Hutton took the trouble to define return, so I decided to use the local definition instead of the one in the Prelude. I changed the last line from "return (x,y)" to "Main.return (x,y)". The new error message is worse:
Couldn't match expected type `Char' against inferred type `[(Char, String)]' In the expression: x In the first argument of `return', namely `(x, y)' In the expression: return (x, y)</blockquote>
Hutton provided explicit type signatures, so I did not expect type issues.
I decided to take another approach. The book has a website that lists errata and provides code listings. The code for chapter eight is at http://www.cs.nott.ac.uk/~gmh/Parsing.lhs
When I read Hutton's code, I noticed that he begins by importing Monad. The code I list above is from chapter 8, pages 75-77 of the book. Monads have not yet been mentioned. The book's index shows that monads aren't mentioned till page 113, in chapter 10. I also notice that in his code, Hutton makes repeated use of a symbol P whose meaning I do not know.