I have occasionally wanted this in Haskell.
let { newtype T = ... } in ...
Compiler signals error if any value of type T "escapes" the scope.
Type synonym types are allowed to escape.
Similarly, let { import ... } . The compiler signals error if any imported type escapes the scope, unless it is also imported in the outer scope.
1 comment :
A slight generalisation would be to only signal an error if the type escapes in a type signature - so it's ok to return them in existentially typed ways
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