Lists of minimum or maximum size

[cueckoo]

Originally opened by @tnarg in cuelang/cue#401

In the List tour I see that you can specify lists of exact length, e.g. 4 * [uint8]. Is it possible to specify a minimum or maximum length for lists? I could specify a uint8 List with between 1 and 4 elements today by enumerating the possibilities like so

1 * [uint8] | 2 * [uint8] | 3 * [uint8] | 4 * [uint8]

For such a small range, this isn't too bad, but for a list between 0 and 50 elements, this becomes exceedingly verbose.

[cueckoo]

Original reply by @tnarg in cuelang/cue#401 (comment)

I can imagine something like

foldJoin([i * [uint8] for i in range(1,50)])

[cueckoo]

Original reply by @myitcv in cuelang/cue#401 (comment)

One way of doing this today is:

package x

x: [...int]
x: [1, 2]
// Not ok
// x: [1, 2, 3]
#checkXLength: len(x) < 3
#checkXLength: true

There have been discussions about how to make this more idiomatic (and indeed make the constraint on the length reusable) but nothing formalised as yet.

[cueckoo]

Original reply by @mpvl in cuelang/cue#401 (comment)

There is a list.MaxItems in the list package. Builtin validators don't currently work correctly, however, as the evaluator doesn't handle non-monotonic constraints yet. This is addressed in the new evaluator work, which will handle such constraints.

Edit: this is now supported (updated to reflect new support in new evaluator)

[cueckoo]

Original reply by @myitcv in cuelang/cue#401 (comment)

There is a list.MaxItems in the list package

I keep forgetting about that package....

[cueckoo]

Original reply by @tnarg in cuelang/cue#401 (comment)

So you'd use list.MaxItems like this?

x: [...int]
#x_length_constraint: list.MaxItems(x, 3)
#x_length_constraint: true

[cueckoo]

Original reply by @mpvl in cuelang/cue#401 (comment)

You would use it as a partial function like this:

a: list.MaxItems(3)

This is not officially supported yet, though, because CUE doesn't handle non-monotonic constraints properly. This is fixed in the new evaluator that I'm currently finishing up.

The way you suggest using it could work, I reckon.

[cueckoo]

Original reply by @myitcv in cuelang/cue#401 (comment)

Update for anyone who sees/reads this discussion: this is now officially supported and works :)

[jbcpollak]

You would use it as a partial function like this:

a: list.MaxItems(3)

Could you elaborate on this a bit please? How would the partial a be applied?

for example:

names: [...string]
a: list.MaxItems(3)
// how to apply a to names?