I do find it very non-obvious. I understand the semantic difference (due to seq), but I don’t understand the typing difference. (So I understand why the compiler can’t automatically eta-expand, but I don’t understand why it would help anything to do so.) Jeff

On Nov 12, 2025, at 1:43 PM, Olaf Klinke via Haskell-Cafe wrote:



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So change lit <$> safeRead n to (\x -> lit x) <$> safeRead n, for example.

Does anyone else find this highly non-obvious? SPJ gives an example https://github.com/ghc-proposals/ghc-proposals/pull/287#issuecomment-5436837... of a context that can indeed distinguish, semantically, a function and its eta expansion. But the fact that, for the benefit of soundness, there must exist contexts where one can be a well-typed program while the other is not, is still mysterious to me.

Can a type system with polymorphism really not do better than this? The discussion thread has some more interesting posts, e.g. https://github.com/ghc-proposals/ghc-proposals/pull/287#issuecomment-5443773... https://github.com/ghc-proposals/ghc-proposals/pull/287#issuecomment-1120461...

At least, thanks to Jaro, the error index now explains when you should eta-expand. https://errors.haskell.org/messages/GHC-83865/

Olaf _______________________________________________ Haskell-Cafe mailing list -- haskell-cafe@haskell.org To (un)subscribe, modify options or view archives go to: Only members subscribed via the mailman list are allowed to post.