Hi,

> length' 1:2:3:[]

is equivalent to:

> (length' 1):2:3:[]

hence the error.

Try:

> length' (1:2:3:[])

Sylvain

Hello all,

I am a rank beginner to functional languages.  Working through Lipovaca's book, up to Chapter 3.

Ok, setup this function in editor and compiled:

length' :: (Num b) => [a] -> b  

length' [] = 0  

length' (_:xs) = 1 + length' xs

skippy@skippy:~$ ghci

GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help

Prelude> :l baby

[1 of 1] Compiling Main             ( baby.hs, interpreted )

Ok, modules loaded: Main.

*Main> length' [1,2,3]

3

*Main> 1:2:3:[]

[1,2,3]

*Main> length' 1:2:3:[]

<interactive>:5:9:

    Could not deduce (Num [a0]) arising from the literal ‘1’

    from the context (Num a)

      bound by the inferred type of it :: Num a => [a]

      at <interactive>:5:1-16

    The type variable ‘a0’ is ambiguous

    In the first argument of ‘length'’, namely ‘1’

    In the first argument of ‘(:)’, namely ‘length' 1’

    In the expression: length' 1 : 2 : 3 : []

*Main> 

Obviously, there is something I don't understand about the apparent non-equivalence of the lists [1,2,3] and 1:2:3:[]I am guessing that the solution is contained in that error message but I can't quite decipher it.  

Thanks for any help.

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