CODEWITHSUNDEEP
haskelltypesghc
J. Abrahamson
J. Abrahamson

Reputation: 74364

Downsides to ScopedTypeVariables

What are the downsides to ScopedTypeVariables, if there are any? Why isn't it on by default? Does it lead to worse inference? Are there edge cases where it fails? Is it significantly harder to implement in GHC?

Upvotes: 10

Views: 366

Answers (2)

Petr
Petr

Reputation: 63379

It's also because it changes the semantics of a program (combined with other extensions). Consider

{-# LANGUAGE RankNTypes #-}
foo :: forall a . (a -> a) -> (a -> a)
foo = bar
  where
    bar :: a -> a
    bar = id

It compiles fine, but with ScopedTypeVariables it even fails to compile.

Upvotes: 11

Thomas M. DuBuisson
Thomas M. DuBuisson

Reputation: 64740

It isn't on by default for the same reason every other extension isn't on by default - it isn't part of the Haskell standard.

As augustss said - this requires more logic in the type checker but I don't think anyone considers it burdensome.

Upvotes: 8

Related Questions