Wherever one can specify a type, let it be possible to use "TYPE_OF(EXPRESSION)" instead. For example, it can be used in type signatures and declarations of type synonyms. Here is an example with syntax similar to Haskell:
A library has functions
f1 :: Int -> Foo;
f1 i = ...
f2 :: Foo -> Int;
f2 f = ...
The user's code importing the library might do something like
type MyFoo = TYPE_OF(f1 undefined);
intermediate :: MyFoo;
intermediate = f1 42;
consume_intermediate :: Int;
consume_intermediate = f2 intermediate;
If a new version of the library were to rename Foo into Bar, the user's code would still work unmodified. (It is irrelevant whether the library exports Foo, later Bar, though not exporting the type would force the user to use TYPE_OF or avoid explicit type signatures, which seems silly.)
This feature could be useful during development if the names of things are not yet decided (bike shedding), but we do know how we want to use them.
The compiler will have to flag errors of circular references of TYPE_OF.
1 comment :
Ken, I'm not seeing how anyone could give a value to `intermediate` or put it in an expression without knowing quite a bit about its type.
Just plain
intermediate = f1 42
achieves the same as you've put. The type signature and TYPE_OF are not helping.
The `asTypeOf` pseudo-function also achieves this.
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