Why can't arguments be levity polymorphic for inline functions?

[Richard Eisenberg]

One significant problem is that {-# INLINE #-} functions are not actually always inlined! Specifically, if an inline-function is not passed all of its arguments, it will not be inlined. This poses a problem for levity-polymorphic functions, and GHC already does some special handling of the few levity-polymorphic primitives, in case they are ever used without all of their arguments.

As for boxing levity-polymorphic arguments: that's plausible, but then I think you've defeated the programmer's intended aim.

The solution to me is some kind of so-called template polymorphism <https://gitlab.haskell.org/ghc/ghc/-/issues/14917#note_151678>. This might work, but it would take a fair amount of design work first.

Richard

> On Oct 7, 2021, at 7:36 PM, Clinton Mead <clintonmead at gmail.com> wrote:
> 
> Hi All
> 
> Not sure if this belongs in ghc-users or ghc-devs, but it seemed devy enough to put it here. 
> 
> Section 6.4.12.1 <https://downloads.haskell.org/~ghc/9.0.1/docs/html/users_guide/exts/levity_polymorphism.html> of the GHC user manual points out, if we allowed levity polymorphic arguments, then we would have no way to compile these functions, because the code required for different levites is different. 
> 
> However, if such a function is {-# INLINE #-} or {-# INLINABLE #-} there's no need to compile it as it's full definition is in the interface file. Callers can just compile it themselves with the levity they require. Indeed callers of inline functions already compile their own versions even without levity polymorphism (for example, presumably inlining function calls that are known at compile time).
> 
> The only sticking point to this that I could find was that GHC will only inline the function if it is fully applied <https://downloads.haskell.org/ghc/9.0.1/docs/html/users_guide/exts/pragmas.html#inline-pragma>, which suggests that the possibility of partial application means we can't inline and hence need a compiled version of the code. But this seems like a silly restriction, as we have the full RHS of the definition in the interface file. The caller can easily create and compile it's own partially applied version. It should be able to do this regardless of levity. 
> 
> It seems to me we're okay as long as the following three things aren't true simultaneously:
> 
> 1. Blah has levity polymorphic arguments
> 2. Blah is exported
> 3. Blah is not inline
> 
> If a function "Blah" is not exported, we shouldn't care about levity polymorphic arguments, because we have it's RHS on hand in the current module and compile it as appropriate. And if it's inline, we're exposing it's full RHS to other callers so we're still fine also. Only when these three conditions combine should we give an error, say like:
> 
> "Blah has levity polymorphic arguments, is exported, and is not inline. Please either remove levity polymorphic arguments, not export it or add an  {-# INLINE #-} or {-# INLINABLE #-} pragma.
> 
> I presume however there are some added complications that I don't understand, and I'm very interested in what they are as I presume they'll be quite interesting. 
> 
> Thanks,
> Clinton
> 
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