Re: [Haskell-cafe] Where does η-equivalence stop?

My best understanding is that eta-equivalence holds only in total languages, and even there without regard to performance. So you can hunt for eta-equivalence trouble anywhere that non-totality or performance comes into play. For example, compare (A) `f (slowFunction 1000)` and (B) `\x -> f (slowFunction 1000) x`. If we map (A) over a list, then `slowFunction 1000` is computed once. If we map (B) over a list, then `slowFunction 1000` is computed for each element of the list. Note that this example is very bare-bones: no extensions, no parametricity-busting `seq`, no compiler flags. All it relies on is a non-strict functional language. I do think compiling this examples is a nice service -- thanks! Richard

On Jun 20, 2022, at 4:38 AM, Hécate wrote:

Hi Café!

I was wondering if anyone knew of a centralised list of occurrences where GHC Haskell stops upholding η-equivalence. I am interested in both type changes and operational semantics. So far I've collected the following things:

* Rank-2 types

* Monomorphism restriction

* The presence of seq

* Non-pedantic bottoms

One source is the GHC Manual¹, and Neil Mitchell pointed me to the list of bugs and limitations of HLint².

If you have other examples, or explanations of the mechanisms at play here, I would be very interested, and intend to upstream those in the GHC manual.

Cheers, Hécate

[¹] https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/bugs.html#ex...

[²] https://github.com/ndmitchell/hlint#bugs-and-limitations

-- Hécate ✨ 🐦: @TechnoEmpress IRC: Hecate WWW: https://glitchbra.in RUN: BSD

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