On Mon, 10 Jan 2022 at 23:04, Greg Ewing <greg.ew...@canterbury.ac.nz> wrote:
>
> Most of the effort of learning to read type expressions, in any
> language, is learning their semantics, which is very different
> from value expressions. Whether they happen to superficially
> resemble other parts of the language is pretty much irrelevant
> to me.
While this is true, what *is* a problem with having a different
language, is that you have two very similar syntactic constructs
("expression" and "type expression") and you have to learn which can
be used where. And that's actually not as trivial as you'd think at
first.
The typing.cast function takes a type expression as its first
parameter. If type expressions are not expressions, syntactically,
then that means that typing.cast(MyType, obj) is *not* syntactically a
function call. So what is it? Say it's a new syntactic element, a
"cast expression". Then the syntax for an expression must be extended
to include cast expressions. And what *is* the syntax of a cast
expression? "cast_literal(type_expr, expr)"? Then what's
"cast_literal"? The value "typing.cast"? What about "from typing
import cast", or "import typing as t" with "t.cast", or "from typing
import cast as typecast", or even "my_cast = getattr(typing, "cast")?
Suddenly there are a huge number of unanswered questions to be
considered, and the answers to those questions need to be learned by
people.
Paul.
PS Having said that, existing type checkers may or may not recognise
all of the possible ways to call the cast function that I described.
Do we want to mandate what type checkers do as part of the language
definition? Do we want to restrict the language definition based on
what 3rd party type checkers can achieve?
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