Hi,
length' 1:2:3:[]
is equivalent to:
(length' 1):2:3:[]
hence the error. Try:
length' (1:2:3:[])
Sylvain On 12/11/2016 21:20, Lawrence Wickert wrote:
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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