On Wednesday 29 September 2004 19:48, Radu Grigore wrote:
> My question is: are there functions of level >= 2 used in practice
> (e.g. (('a -> 'b -> 'a) -> 'a -> 'b list -> 'a) -> 'c)?

I've just had a look through some real programs that I've written and the 
answer is definitely yes. I use them quite a lot. For >2 they are mainly 3, 
sometimes 4 and I haven't seen any >4.

> If so, are 
> there any typical ones that appear in many applications (maybe not as
> widespread like map & company but at least of comparable usefulness)?

I seem to use them when I write generic functions which are later specialised.

> One example of a level 2 function (stolen from a recent post by Jon
> Harrop) is this:
>   let sum fold = fold (+);;

Funny to think that this function is still state-of-the-art Java and C++. ;-)

Cheers,
Jon.

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