Re: Gradual 'tree' typing and C++

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Jakub Jelinek <[email protected]> writes:

> On Tue, Jul 21, 2026 at 08:34:09PM +0200, Arsen Arsenovi=C4=87 wrote:
>> Checks-and-uses of TREE_CODEs would also need to become (explicit)
>> fallible conversion functions, i.e.
>>=20
>>   if (TREE_CODE (t) =3D=3D A) use t;
>>=20
>> ... becomes:
>>=20
>>   if (auto tA =3D try_as_tree_or_whatever<A> (t)) use tA;
>>=20
>> ... or such.  (I'd have preferred if the 'tA' could shadow 't', but
>> unfortunately if it were to shadow 't' in the then branch, it'd also
>> have to shadow it - a lot less usefully - in the else branch; that's not
>> an issue in this trivial example but is in general)
>
> I'm sorry, but this is not an improvement, it is an exact opposite of
> that.

Interesting, I thought that this specific example was overly favourable
to the idea of in-language gradual typing.

It's the trivial case where one check is replaced by another check whose
result also gets encoded in the type system/current scope.

Such a case - converting a check into a check + extra information in the
variable type pretty much mechanically - seemed like strictly an
improvement to me.

In the languages I was mentioning as a reference, as well as in some
others like Kotlin, the check alone would mean that 't' gets a narrower
type definition in the scope of the then stmt, but this isn't possible
in C++, sadly.

Note that, at least initially, calling a function that receives a typed
tree with an untyped tree should be possible (and result in the compiler
automatically adding a runtime check), and calling a function that
receives an untyped tree with a typed tree should probably always be
possible.  This is indeed what would allow for a gradual change.

What was problematic in that example?
=2D-=20
Arsen Arsenovi=C4=87

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