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

If you add a let binding: let x = slowFunction 1000 in f x \y -> let x = slowFunction 1000 in f x y I believe GHC may optimise both functions to only compute the `slowFunction 1000` once due to the full laziness/let floating optimisation. So then the eta-equivalence holds again. Maybe the full laziness optimisation should be disabled by default to make performance more predictable and to make it even more obvious that eta-equivalence doesn’t hold. See https://gitlab.haskell.org/ghc/ghc/-/issues/8457 Cheers, Jaro

On 28 Jun 2022, at 11:57, Richard Eisenberg wrote:

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

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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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