> If a function has type declaration `f :: A -> B -> C` you would expect that its body only gets evaluated when two arguments are actually supplied. That’s not

> If a function has type declaration `f :: A -> B -> C` you would expect that its body only gets evaluated when two arguments are actually supplied. That’s not correct, regardless of η expansion. The core of the issue is that this function really takes one argument, regardless of how in practice it is used as if it had two. > the expressions `seq f (putStrLn "hello")` and `seq (f undefined) (putStrLn "hello")` should both not evaluate anything and cause the string hello to be printed rather than raising an exception. That’s not how `seq` works, as per language report. The definition is very simple, and does not take into account any kinds of types (such as treating functions differently). The report demands that `seq ⊥ x = ⊥` [1], so if the first argument is ⊥, you get a crash. Whether `x` is also evaluated in this context is up to the compiler actually, but what certainly cannot happen is that a "hello" is printed, because that would require a non-⊥ value to be passed to the surrounding IO context, but the whole `seq` is ⊥, so that’s not possible. In GHC, seq is essentially a tag that helps the strictness analyzer, and hence the optimizer. There is one subtlety in GHC that makes it not respect η expansion, and that is strictness properties, namely that `seq ⊥ () = ⊥`, but `seq (\x -> ⊥ x) () = ()`, were you referring to that? (In my opinion this is a violation of the Haskell Report.) > This often occurs in cases where the above hypothetical function `f` requires an intermediate value that's expensive to compute but only relies on the first of its two arguments. Lookup tables work that way, yes! I’m using this technique for walking a certain distance on a Bezier curve for example (Code at [2], result picture at [3]) Call that academic and I’ll show you the DIN A1 sized CNC machine I control using that logic ;-P [1]: https://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-1260006.2 https://www.haskell.org/onlinereport/haskell2010/haskellch6.... [2]: https://github.com/quchen/generative-art/blob/master/src/Geometry/Bezier.hs#L236 https://github.com/quchen/generative-art/blob/master/src/Geo... [3]: https://quchen.github.io/generative-art/generative-art-0.1.0.0/Geometry-Bezier.html#v:bezierSubdivideEquidistant https://quchen.github.io/generative-art/generative-art-0.1.0...