[Haskell-cafe] I seem to constantly abuse TypeFamilies; what do i really want?

[Eric Walkingshaw]

>
>  That's a great point, thanks. I'll have to remember to interrogate
>> myself on that next time I find myself reaching for this pattern. I wonder
>> if your version can be made to work for functions of any arity?
>>
>
Since functions get their arguments from the stack in reverse order, this
implementation actually already supports functions of any arity if you just
follow it with an appropriate number of `F2 ($)` commands.

First, here's a nicer eval function:

    eval :: Prog () (a,())
    eval p = fst (prog p ())

And here's how to apply the 3-ary function `zipWith` and the 4-ary function
`zipWith3`:

    > eval (Push [1..3] :. Push [4..6] :. Push (,) :. F2 zipWith :. F2 ($)
:. End)
    [(4,1),(5,2),(6,3)]

    > eval (Push [1..3] :. Push [4..6] :. Push [7..9] :. Push (,,) :. F2
zipWith3 :. F2 ($) :. F2 ($) :. End)
    [(7,4,1),(8,5,2),(9,6,3)]

-Eric
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