How to define an arbitrary arity function in Haskell, which includes an arity of 0?

Question

My current approach to define a function of arbitrary arity is below, with A being an accumulator, E being the input argument type, and R being the result type.

combine :: A -> E -> A

class X r where
    foo :: A -> E -> r

instance X R where
    foo :: A -> E -> R


instance X r => X ( E -> r ) where
    foo :: A -> E -> E -> r
    foo ( a :: A ) ( x :: E ) =
        foo ( a `combine` e :: A )

doFoo = foo emptyA

But the minimum arity of foo is 1. The minimum for foo is still A -> E -> R, and doFoo is E -> R. I'd also like to have doFoo :: R. How?

Rotsor · Accepted Answer · 2011-07-11T05:14:41.517

10

What about

class X r where
    foo :: A -> r

instance X r => X (E -> r) where
    foo :: A -> E -> r
    foo a e = foo (combine a e)

?

You may want to have a look at the PrintfType instances. It's only because of them that I was able to provide an answer.

edited Jul 11 '11 at 05:14

answered Jul 11 '11 at 05:05

Rotsor

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You don't. This question may be useful for other people. – hammar Jul 11 '11 at 06:06
1

Another useful package to look at is quickcheck. It also provides functions with variadic arguments. – Masse Jul 11 '11 at 07:08

How to define an arbitrary arity function in Haskell, which includes an arity of 0?

1 Answers1