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

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(Discussing code indentation for nice cut-and-paste.)
 
 
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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
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-- 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)
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-- 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 code.
  
   
  +
== Problems ==
⚫
(P.S. I am really enjoying this article! It provides a very nice practical introduction to Haskell. [[User:Mforbes|Mforbes]] 09:54, 17 April 2006 (UTC))
  
  +
  +
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
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eof
  +
return dirs
  +
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
  ⚫
eof
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return dirs
  +
  +
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)
  +
  +
main = do input <- getContents
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putStrLn $ "DEBUG: got input " ++ input
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-- 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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