Difference between revisions of "Talk:Hitchhikers guide to Haskell"

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== Code Indentation: ==
  
== Code Indentation: ==
  +
Haskell code should be indented using the <haskell> tag:
−
Code is most easily specified by indenting it by a space in the wiki source:
  
   
  +
http://www.haskell.org/haskellwiki/HaskellWiki:Syntax_highlighting
−
-- This is some code.
  
−
sq x = x*x
  
   
  +
This also allows for proper cut and paste properties.
−
If two spaces are used to indent, then the code is visually indented:
  
   
  ⚫
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)
−
-- Indented code.
  
⚫
sq x = x*x
  
   
−
However, if two spaces are used, then cutting and pasting the code sample will insert a spurious space that breaks the 2D Haskell layout. Unless there is a way of indenting code visually while preserving the cut an paste layout properties, it seems that one should only use a single space to indent wiki code.
  
   
  +
== Problems ==
−
(I did not change the interactive code as it is not likely to be cut and pasted over multiple lines. Visually, however, it might be good to remove the extra space here as well...)
  
   
  +
When I try to compile, or load into ghci, the following code I get get this error:
⚫
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)
  
  +
  +
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
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eof
  +
return dirs
  +
Failed, modules loaded: none.
  +
  +
And I panic.
  +
  +
Code:
  +
<haskell>
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module Main where
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  +
import Text.ParserCombinators.Parsec
  +
  +
data Dir = Dir Int String deriving Show
  +
  +
parseInput =
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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
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putStrLn $ "DEBUG: got input " ++ input
  +
-- compute solution and print it
  +
</haskell>
  +
  +
/Liem
  +
  +
== Solution ==
  +
I solved the above problem by adding a signature to one function:
  +
  +
<haskell>
  +
parseInput :: Parser [Dir]
  +
parseInput = ...
  +
</haskell>
  +
  +
If someone explained ''why'' the signature is required, I could die happy.
  +
  +
[[User:Mgm7734|Mgm7734]] 23:29, 30 July 2011 (UTC)
  +
  +
Its because of the [[Monomorphism_restriction]]
  +
  +
== Knapsack problem? ==
  +
  +
Surely this is actually an example of bin packing? No?

Latest revision as of 17:19, 4 June 2012

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)


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

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

Knapsack problem?

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

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