Talk:Hitchhikers guide to Haskell
(→Knapsack problem?: new section)
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Latest revision as of 17:19, 4 June 2012
 1 Code Indentation:
Haskell code should be indented using the <haskell> tag:
This also allows for proper cut and paste properties.
P.S. I am really enjoying this article! It provides a very nice practical introduction to Haskell. Thanks! Mforbes 10:08, 17 April 2006 (UTC)
 2 Problems
When I try to compile, or load into ghci, the following code I get get this error:
cd-fit.hs:8:13: No instance for (Text.Parsec.Prim.Stream s m Char) arising from a use of `many' at cd-fit.hs:8:13-27 Possible fix: add an instance declaration for (Text.Parsec.Prim.Stream s m Char) In a stmt of a 'do' expression: dirs <- many dirAndSize In the expression: do dirs <- many dirAndSize eof return dirs In the definition of `parseInput': parseInput = do dirs <- many dirAndSize eof return dirs Failed, modules loaded: none.
And I panic.
module Main where import Text.ParserCombinators.Parsec data Dir = Dir Int String deriving Show parseInput = do dirs <- many dirAndSize eof return dirs dirAndSize = do size <- many1 digit spaces dir_name <- anyChar `manyTill` newline return (Dir (read size) dir_name) main = do input <- getContents putStrLn $ "DEBUG: got input " ++ input -- compute solution and print it
 3 Solution
I solved the above problem by adding a signature to one function:
parseInput :: Parser [Dir] parseInput = ...
If someone explained why the signature is required, I could die happy.
Mgm7734 23:29, 30 July 2011 (UTC)
Its because of the Monomorphism_restriction
 4 Knapsack problem?
Surely this is actually an example of bin packing? No?