Proposal:
=========
The `type select` statement should be removed in favor of `select x.type`,
which works today and provides nearly the same functionality.
Rationale:
=========
- The `type select` statement is basically redundant. For example:
var x = 5;
type select x {
when int do writeln("int");
when real do writeln("real");
}
can instead be written as:
var x = 5;
select x.type {
when int do writeln("int");
when real do writeln("real");
}
This works in 1.10.0, and maybe in earlier releases.
- Due to a bug in the current implementation of `type select`,
there can be only one `type select` statement in a given scope.
Caveats:
========
- The behavior of the two statements isn't identical.
`type select` uses the function resolution mechanism to choose the
matching `when` clause, while `select` uses the equality operator (==).
For example:
var x: bool = true;
type select x {
when int do writeln("int");
otherwise writeln("other");
}
This will print "int", because a variable of type `bool` can be implicitly
converted to an `int`, following the function resolution rules.
On the other hand, the `select x.type` variant:
select x.type {
when int do writeln("int");
otherwise writeln("other");
}
will print "other", because x.type is `bool`, and `bool == int` evaluates
to false.
The `type select` behavior can be achieved using functions. For example:
proc f(a: int) { writeln("int"); }
proc f(a) { writeln("other"); }
f(x);
or via some hypothetical future type-coercion introspection abilities:
if isImplicitlyConvertibleToInt(x) then writeln("int") else
writeln("other");
Plan:
=====
If there are no major objections to this proposal, `type select` will
become a syntax error in the next release of chapel (with an error message
suggesting `select .type`). In the following release, the parser won't
recognize `type select`, and will generate a generic syntax error.
Does anyone have any arguments against removing `type select`?
Are there any use-cases that `type select` covers that can't
be handled another way?
Thanks,
Sean
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