Add ability to define custom predicate functions

[neetle]

Hi!

I'm currently working on a project that would like to use this as the primary language to build filters for it's frontend.

At the moment, we're hoping to allow users to write queries like Nodes | filter(.Name=="cat"). We're currently facing an issue though, as Nodes isn't a slice. This leads to issues like array.Len() causing panics in the VM every time we hit an OpBegin.

As we don't control the layout/go type of the data we're passing into expr, would you be against defining some way to implement functions that take custom predicates?

Looking forwards, this could also be a good place to look into support for the changes proposed in golang/go#61898.

[antonmedv]

Yes, it is possible to implement next expression, even if Nodes is not an array.

Nodes | filter(.Name=="cat")

To do it you can create a patcher which will be wrapping Nodes inside filter into a helper function. The helper function will transform Nodes to an array.

After the patcher resulting code should look like this:

nodesToArray(Nodes) | filter(.Name=="cat")