[Haskell-cafe] Counterintuitive ScopedTypeVariables behaviour

[Alexis King]

Adding to my previous email, I’ve noticed that the behavior I’ve described appears to be inconsistent with the documentation. From the GHC 8.10.1 User’s Guide § 9.17.1, Lexically scoped type variables » Overview <http://downloads.haskell.org/ghc/8.10.1/docs/html/users_guide/glasgow_exts.html#id59>:

> The design follows the following principles
> A scoped type variable stands for a type variable, and not for a type. (This is a change from GHC’s earlier design.)
> Furthermore, distinct lexical type variables stand for distinct type variables. This means that every programmer-written type signature (including one that contains free scoped type variables) denotes a rigid type; that is, the type is fully known to the type checker, and no inference is involved.
> Lexical type variables may be alpha-renamed freely, without changing the program.
Emphasis mine. This suggests that your program should not be accepted (and that GHC shouldn’t work the way I describe, even if that’s how it works in practice today), so either the current implementation is buggy, or the documentation is buggy. I am not sure which.

I have opened GHC issue #18591 <https://gitlab.haskell.org/ghc/ghc/-/issues/18591> to track this inconsistency.

Alexis
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