在 2010年 1月 17日星期日 11:02:59，michael rice 寫道:
> I don't see anything wrong with this function, which just subtracts 1 from
> the first element of an Int list (if there is a first element).
>
> Michael
>
> My function:
>
> dropFirst :: [Int] -> [Int]
> dropFirst [] = []
> dropFirst (x:xs) = (x-1) : xs
>
> My output:
> > :l dropfirst
>
> [1 of 1] Compiling Main    ( dropfirst.hs, interpreted )
> Ok, modules loaded: Main.
> *Main> dropFirst [3 4 5 6]

Use dropFirst [3,4,5,6], I think.

>
> <interactive>:1:11:
>    No instance for (Num (t -> t1 -> t2 -> Int))
>    arising from the literal `3' at <interactive>:1:11-17
>    Possible fix:
>    add an instance declaration for (Num (t -> t1 -> t2 -> Int))
>    In the expression: 3 4 5 6
>    In the first argument of `dropFirst', namely `[3 4 5 6]'
>    In the expression: dropFirst [3 4 5 6]
> *Main>
>
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