Special keyword to define mapping functions in expressions

[LeaVerou]

We've often struggled with how to let authors define inline functions in expressions, the concept of an arrow function seeming too out of reach for them.
This would be useful for many things:

• Mapping operations
• Custom sorting
• Filtering

Note that unlike Coda, we don't need things like CountIf() or All() since we can always do things like count(foo > 2) > 1, but they may be useful as sugar.

Coda’s formula language solves this in an interesting way: They allow for a special CurrentValue keyword, which essentially represents the closest list item. In Mavo’s syntax, presumably this could be $value or $current?

There is the problem of referring to outer currentvalues, which they solve with a special scoping function. I think that's suboptimal, I can think of two alternative ways to deal with it, which can be combined:

• A special $outer keyword. $outer.$value would be the outer $value, and things like $outer.$outer.$value would also be possible
• Supporting arrow functions as well, and framing it as "$value is the default parameter name, but you can provide your own name by using this syntax".

[DmitrySharabin]

They allow for a special CurrentValue keyword, which essentially represents the closest list item. In Mavo’s syntax, presumably this could be $value or $current.

Correct me if I’m wrong, but we already have $item, which represents exactly what you described—the closest list item. Can’t we simply re-use it? What am I missing?

[LeaVerou]

They allow for a special CurrentValue keyword, which essentially represents the closest list item. In Mavo’s syntax, presumably this could be $value or $current.

Correct me if I’m wrong, but we already have $item, which represents exactly what you described—the closest list item. Can’t we simply re-use it? What am I missing?

$item (and $this) are fixed within the same execution: they refer to data from the nearest node that matches certain criteria. $value has no meaning outside iterative functions, which loop over a list and $value refers to the current value each time.

Some examples (assume we had map() and sort() functions):

Hypotenuse using numbers in a list:

sqrt(sum(map(numbers, pow($value, 2))))

Filter a list and only keep even numbers:

filter(numbers, $value mod 2 = 0)

Single line pivot table: From a list of flights, to stats about outbound airports:

map(flights by outbound, group(outbound: $value.outbound, count: count($value.$items))

This can already be expressed with current syntax like so:

filter(numbers, numbers mod 2 = 0)

or

numbers where numbers mod 2 = 0

Similarly, it enables patterns like

people.sort($value.name, "asc")

(map each person to their name, then sort based on that)

[DmitrySharabin]

Makes sense. Thank you!