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Talk:Hitchhikers guide to Haskell

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(<haskell></haskell> is the correct solution.)
(Solution)
 
(5 intermediate revisions by 3 users not shown)
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P.S. I am really enjoying this article!  It provides a very nice practical introduction to Haskell.  Thanks!  [[User:Mforbes|Mforbes]] 10:08, 17 April 2006 (UTC)
 
P.S. I am really enjoying this article!  It provides a very nice practical introduction to Haskell.  Thanks!  [[User:Mforbes|Mforbes]] 10:08, 17 April 2006 (UTC)
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 +
 +
== Problems ==
 +
 +
When I try to compile, or load into ghci, the following code I get get this error:
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 +
    cd-fit.hs:8:13:
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    No instance for (Text.Parsec.Prim.Stream s m Char)
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      arising from a use of `many' at cd-fit.hs:8:13-27
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    Possible fix:
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      add an instance declaration for (Text.Parsec.Prim.Stream s m Char)
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    In a stmt of a 'do' expression: dirs <- many dirAndSize
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    In the expression:
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        do dirs <- many dirAndSize
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          eof
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          return dirs
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    In the definition of `parseInput':
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        parseInput = do dirs <- many dirAndSize
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                        eof
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                        return dirs
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    Failed, modules loaded: none.
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And I panic.
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Code:
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<haskell>
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module Main where
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import Text.ParserCombinators.Parsec
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data Dir = Dir Int String deriving Show
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parseInput =
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  do dirs <- many dirAndSize
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    eof
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    return dirs
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dirAndSize =
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  do size <- many1 digit
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    spaces
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    dir_name <- anyChar `manyTill` newline
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    return (Dir (read size) dir_name)
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main = do input <- getContents
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          putStrLn $ "DEBUG: got input " ++ input
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          -- compute solution and print it
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</haskell>
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/Liem
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== Solution ==
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I solved the above problem by adding a signature to one function:
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<haskell>
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parseInput :: Parser [Dir]     
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parseInput = ...
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</haskell>
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If someone explained ''why'' the signature is required, I could die happy.
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 +
[[User:Mgm7734|Mgm7734]] 23:29, 30 July 2011 (UTC)
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 +
Its because of the [[Monomorphism_restriction]]
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 +
== Knapsack problem? ==
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 +
Surely this is actually an example of bin packing? No?

Latest revision as of 17:19, 4 June 2012

Contents

[edit] 1 Code Indentation:

Haskell code should be indented using the <haskell> tag:

http://www.haskell.org/haskellwiki/HaskellWiki:Syntax_highlighting

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)


[edit] 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.

Code:

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

/Liem

[edit] 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

[edit] 4 Knapsack problem?

Surely this is actually an example of bin packing? No?