Cover function expressions as function parameters

[jg-rp]

These additional test cases cover using function expressions as function parameters, plus an additional length() case mentioned in the spec.

Note that it's not currently possible to test count() and value() with function expressions as parameters using standard function extensions alone. None of the standard function extensions return a NodesType.

This PR partially replaces #51. See also the conversation on PR #51.

[f3ath]

I like this addition. @gregsdennis care to merge?

[gregsdennis]

These look good, but I'd like to see some examples where value() returns Nothing (the nodelist is either empty or contains multiple elements).

For example, with this data

{
  "c": "cd",
  "values": [
    {
      "a": "ab"
    },
    {
      "c": "d"
    },
    {
      "a": null
    }
  ]
}

value(@.a) returns Nothing for {"c": "d"} and null for {"a": null"}. length() should then return Nothing for both of these values.

value($..c) returns Nothing because there are multiple matching locations.

Finally, Nothing == Nothing returns true, meaning that the last two elements are returned in the final result:

[
  {
    "c": "d"
  },
  {
    "a": null
  }
]

[gregsdennis]

@glyn can you verify my logic ☝️ please? This is what my lib currently does.

[glyn]

@glyn can you verify my logic ☝️ please? This is what my lib currently does.

LGTM

[jg-rp]

These look good, but I'd like to see some examples where value() returns Nothing (the nodelist is either empty or contains multiple elements).

Hi @gregsdennis, I've added a couple more cases which I believe cover the behaviour demonstrated by your example.

[gregsdennis]

These work for me.