[Haskell-cafe] Newbie questions

Duncan Coutts duncan.coutts at worcester.oxford.ac.uk
Thu Jul 1 12:29:37 EDT 2004

On Thu, 2004-07-01 at 17:01, Crypt Master wrote:
> I consider myself a pretty good imperative programmer, been at it for 
> decades, and am tryibng my hand at Haskell now. Unfrituantly I am not 
> "getting" it all that quickly despite having the multimedia haskell book.


> The take operator doesnt work on this. from hugs:
> HAGA> take 5 gaSolutionSpace [1,2,3,4,5]
> ERROR - Type error in application
> *** Expression     : take 5 gaSolutionSpace [1,2,3,4,5]
> *** Term           : take
> *** Type           : Int -> [e] -> [e]
> *** Does not match : a -> b -> c -> d

What this error message is telling you is this:
      * The 'take' function wants two parameters, an Int and a list of
        some type '[e]'. It will give you back a list of the same type
      * You have given it three parameters, or to put it another way you
        are using 'take' as if it had type 'a -> b -> c -> d' (for some
        a,b,c & d)

The solution then is to just pass two arguments. How do you do that?
In what you've written can you see that you're passing 3 arguments?
take 5 gaSolutionSpace [1,2,3,4,5]

People say function application in Haskell is written without brackets
but this can be misleading, here you do need brackets to indicate that
'gaSolutionSpace [1,2,3,4,5]' is one argument and not two. So you should
take 5 (gaSolutionSpace [1,2,3,4,5])

Now there are just two parameters, where the second parameter is another
expression rather than a simple variable or constant.

If your gaSolutionSpace takes a list type and returns a list type then
this will be well typed, and hugs will not complain.


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