Folding Curried Functions in Scala [duplicate]

Question

Possible Duplicate:
Applying an argument list to curried function using foldLeft in Scala

Consider:

val f = (i: Int, j: Int, k: Int, l: Int) => i + j + k + l

Because one can do this:

f.curried.apply(1).apply(2).apply(3).apply(4)

It's easy to fall into the trap of trying this:

List(1, 2, 3, 4).foldLeft(f.curried) { (fs, e) => fs.apply(e) }

However, the B parameter in the fold changes type once one applies an argument to it. In this example, the first iteration would change from a (Int) => (Int) => (Int) => (Int) => Int to a (Int) => (Int) => (Int) => Int.

Question: How to solve this?

I don't think this makes sense. In (fs, e) => fs.apply(e) you are actually using a different type for fs on every iteration and it's a different apply function. I don't see how it could work. — Gabe
– Gabe, Commented Jan 9, 2012 at 16:29
I think the problem is that it's not entirely clear what is being looked for here. The problem suggested in the title - folding curried functions - is never going to work because of the problem you identify. So what's the underlying problem? Applying a list of parameters to a function? — Submonoid
– Submonoid, Commented Jan 9, 2012 at 16:34

Submonoid · Accepted Answer · 2012-01-09 16:47:25Z

You should be able to do this with an HList - see http://apocalisp.wordpress.com/2010/07/08/type-level-programming-in-scala-part-6b-hlist%C2%A0folds/ for an example of building fold functions over heterogenous lists.

Edit - hbatista's link in the comment above provides a full solution doing precisely this.

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