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I think in Haskell you need to add: {-# LANGUAGE UndecidableInstances #-} to get an Undecidable type system.
by trissylegs 5y ago
I think in Haskell you need to add:
{-# LANGUAGE UndecidableInstances #-}
to get an Undecidable type system.
- arianvanp 5y agoThere are other language extensions to make the haskell type system undecidable. The classic example is `RankNTypes` E.g. you can proof that type systems of rank n >= 3 are undecidable. [0] I think that's why haskell has both Rank2Types and RankNTypes as a language extension. So you can still use higher rank types without running into decidability problems up to rank 2. [0] https://dl.acm.org/doi/10.1145/182409.182456 https://dl.acm.org/doi/10.1145/182409.182456