W/W should transform such a function into one who takes one less argument removing any runtime overhead at least for fully applied functions. I suppose your suggestion is then if we an expression`f x` where bar takes multiple arguments, but doesn't use the current argument then GHC should: * Inspect f, check if the first argument to f is used * If we can determine it isn't used instead of creating a PAP capturing `f` and `x` instead only capture `f` and record this in the PAP closure somehow. * Once the PAP is fully applied pass a dummy argument instead of `x` to f. If f is a known call that seems doable, although adding a bitmap to paps might require us to increase the size of all PAP closures, making this optimization less useful. If `f` is a unknown function there is currently no way to get absent/used info for it's arguments at runtime. And changing that would be a major change which seems unlikely to pay off. So I think this would be theoretically possible, but it would rarely pay off. Also do you have an example where `(const a) b` leads to stupid thunks? It seems to me const should always be inlined in such a case, avoiding a PAP allocation. Am 12/04/2022 um 23:02 schrieb David Feuer:

Suppose `f` doesn't use its first argument. When forming the thunk (or partial application) `f a`, we don't need to record `a`. What if instead of arity, we store a bitmap used/absent arguments, terminated by a 1 bit? Could we then get rid of "stupid thunks" like `(const a) b`?

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