[Haskell-cafe] argument counts...

Arjen arjenvanweelden at gmail.com
Mon Nov 20 19:42:07 UTC 2017

On Mon, 2017-11-20 at 11:13 -0600, Gregory Guthrie wrote:
> I have a simple function:
>        revFn (x:xs) = (revFn xs) . (x:)
> Of course GHCi correctly infers the type as:   revFn :: [a] -> [a] ->
> c
> Adding the base case:
>     revFn [] xs = xs
> Now gives an error;
>   “Equations for ‘revF’ have different numbers of arguments”
> Of course this can be “fixed” by either adding the cancelled argument
> to the first clause, or converting the base case to only have one
> explicit argument, and a RHS of a lambda or identity function.
> But since the interpreter already correctly inferred that the first
> clause has two arguments (with only one explicit), why does it then
> ignore this and give an error when the second clause shows two
> explicit arguments? The types are all correct in either case – why
> require explicit arguments?
> Or, perhaps I am missing something simple?
The semantics are based on Term Rewriting Systems. The number of
arguments determines when the left side gets rewritten (evaluated) to
the right side. This makes a difference in time and space properties. I
think that is why Haskell (GHC) makes it explicit, and requires all
parts of the function definition to have the same number of arguments.

hope this helps, Arjen

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