[Haskell-cafe] General function to count list elements?

[Eugene Kirpichov]

What should

count (\x -> x) (replicate 10 (\y -> if 1==1 then y else undefined))

return?

2009/4/18 michael rice <nowgate at yahoo.com>:
> Is there a general function to count list elements. I'm trying this
>
> count :: a -> [a] -> Int
> count x ys = length (filter (== x) ys)
>
> with this error upon loading
>
> =============
>
> [michael at localhost ~]$ ghci count
> GHCi, version 6.10.1: http://www.haskell.org/ghc/  :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer ... linking ... done.
> Loading package base ... linking ... done.
> [1 of 1] Compiling Main             ( count.hs, interpreted )
>
> count.hs:2:29:
>     Could not deduce (Eq a) from the context ()
>       arising from a use of `==' at count.hs:2:29-32
>     Possible fix:
>       add (Eq a) to the context of the type signature for `count'
>     In the first argument of `filter', namely `(== x)'
>     In the first argument of `length', namely `(filter (== x) ys)'
>     In the expression: length (filter (== x) ys)
> Failed, modules loaded: none.
> Prelude>
>
> =============
>
> Not sure what it's trying to tell me other than I need an (Eq a) somewhere.
>
> Michael
>
>
>
>
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>



-- 
Eugene Kirpichov
Web IR developer, market.yandex.ru