MonoLocalBinds and hoopl

[Edward Z. Yang]

I ran into some more code like this, and I realized there was something
pretty important: the majority of let-bindings do not have any free varaibles.
They could very well be floated to the top level without having to make any
source level changes.

So maybe let should be generalized, if no free variables are captured.
Some food for thought.

Cheers,
Edward

Excerpts from Edward Z. Yang's message of Thu Dec 09 10:28:20 -0500 2010:
> Hello all,
> 
> Here's an experience report for porting hoopl to manage MonoLocalBinds.  The
> Compiler.Hoop.XUtil module has a rather interesting (but probably common) style of code
> writing, along the lines of this:
> 
> fbnf3 (ff, fm, fl) block = unFF3 $ scottFoldBlock (ScottBlock f m l cat) block
>     where f n = FF3 $ ff n
>           m n = FF3 $ fm n
>           l n = FF3 $ fl n
>           FF3 f `cat` FF3 f' = FF3 $ f' . f
> 
> f, m, l and cat are polymorphic functions that are only used once in the
> main expression, and are floated outside to improve readability.  However, when
> MonoLocalBinds is turned on, these all become monomorphic and the definitions
> fail.  In contrast, this (uglier) version typechecks:
> 
> fbnf3 (ff, fm, fl) block = unFF3 $ scottFoldBlock (ScottBlock (FF3 . ff) (FF3 . fm) (FF3 . fl) (\(FF3 f) (FF3 f') -> FF3 $ f' . f)) block
> 
> One suggestion that I had was that we should generalize local bindings that
> are only used once, but Marlow pointed out that this would make the typechecker
> more complex and I probably would agree.
> 
> As a userspace developer, I have two options:
> 
>     1. Bite the bullet and put in the polymorphic type signatures (which
>        can be quite hefty)
>     2. Inline the definitions
>     3. Move the polymorphic functions into the global namespace
> 
> (3) and (2) are not so nice because it breaks the nice symmetry between these
> definitions, which always define f, m, l for the many, many definitions in
> Hoopl of this style.
> 
> Cheers,
> Edward
>