Ordering of arguments is an age-old programming challenge (creating many
bugs). Type checking and pattern matching help, but are also limited. We
currently have two indirect ways of having named arguments, but both have
challenges:
1. Keyword lists — pattern matching must match the exact order of the
listed arguments, which doesn't help the issue here which is ordering, let
alone optional arguments, leading to extraneous in-function calls to
Keyword.get.
2. Maps — while this supports optional arguments, it has the overhead of
creating and destructing a map for a simple function call. Benchmarking
this vs ordered arguments shows it's 1.79x slower than simply using ordered
arguments. That's an overhead that builds up on every function (I didn't
benchmark keyword lists, sorry).
3. Existing syntax to handle named arguments as keywords should still be
handled, for backwards compatibility, making it harder to do this sort of
thing.
To preface the proposal, I realize this may simply not be possible with the
limitations of the parser/compiler that we have today.
Proposal:
Add a compile-time named/pinned argument syntax in the function declaration
head, which allows the naming of arguments as if it were a keyword list,
but instead, the keys are mapped to the variable names/pins and, if
necessary, are rearranged to keep the ordering correct.
This will not work with conventional keyword arguments, where one expects a
keyword list—if using the named/pinned syntax, keyword arguments may not be
used. It's one or the other, not both.
If this named syntax exists on a function head, then a calling function may
use the `name: value` syntax and it will align the values to the named
argument at compile time (no keyword lists are used at runtime).
Example:
Using `&` as a reference for the naming (or possibly if not that, the
asterisk):
def do_a_thing(&new, &old)
Would accept any of these calls, and in all cases the variable values
within the called function would align properly:
do_a_thing("newer value", "older value")
do_a_thing(new: "newer value", old: "older value")
do_a_thing(old: "older value", new: "newer value")
Optional named arguments in the same manner as optional arguments today:
def do_a_thing(&new, &old, &optional \\ "extra info")
Optional ideas:
1. If rearranging the arguments at compile time is not easily feasible,
it could simply just raise a compiler error when out of order, and then
strip the names when they are properly ordered.
2. If using named/pinned arguments, always require them to be named
(thus, the first example above would be invalid). However, this comes with
its own challenge, notably what about when using pipelines? Perhaps just
allow that.
3. If it's too challenging to use the same syntax as keyword lists,
another naming convention could be used. It's just ... uglier. Perhaps if
using `*` instead it would be on both sides (function head, and calling
function), such as:
def do_a_thing(*new, *old)
...
do_a_thing(*new: "newer value", *old: "older value")
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