shortcut syntax for partial function application

Currently, it’s easy enough, but kinda verbose, to partially apply a function:

value formatHex = (Integer n) => Integer.format(n, 16);

The discussion in #6615 raised the possibility of having a shortcut syntax for the special but very common case where we’re partially applying a function of multiple parameters and obtaining a function with exactly one parameter.

Basic proposal

The proposed syntax would be, approximately:

value formatHex = Integer.format(it, 16);

Where it is a “magic identifier” or “soft” keyword. (That is, if there is nothing named it declared in the current scope, it would be given this special interpretation. Furthermore, a warning would be produced wherever the name it is declared.)

A reasonable objection to this proposal is that it is much too much of a special case:

• it doesn’t handle arbitrary expressions, and
• it only handles the limited case of exactly one argument.

I think these objections are reasonably dispatched by noting that:

• Any attempt to make this work for arbitrary expressions runs into the problem of nesting, as discussed in #7190.
• The single-argument case is, I believe, overwhelmingly the most common case, especially in the context of pipelining discussed in #6615.

Possible extension

An open question is whether this feature could be extended to the receiving instance of a method invocation, in contexts where the receiving instance type can be inferred, for example:

strings.map(it.initial(3));

would mean:

strings.map((s) => s.initial(3))

and:

string |> it.initial(3)) |> list.add

would mean:

string |> ((s) => s.initial(3)) |> list.add

I’m not sure about that, but it’s worth considering.

[xkr47]

For the record, I dislike the keyword it, since I use it for iterators 😃

[luolong]

After seeing the version with soft keyword ‘it’, I tend to agree to a degree with @jensli here that the keyword ‘it’ does not really bring out the fact that we are dealing with a partial application here.

If we stay with keyword approach, I would much prefer it if ‘value’ could be used - along with the keyword like treatment in the rest of the language it is much less likely that it would be confused with a local variable name than ‘it’.

Other than that I do have a preference for using ‘_’ (underscore) as a partial application wildcard. I’ve always seen it as a meaningless sigil to replace some variable name whenever I mean to say “I don’t care what is the input; Just do this”