MonoLocalBinds and hoopl

Simon Peyton-Jones simonpj at
Fri Dec 10 13:52:20 CET 2010

Yes, argument to higher rank functions are probably the top reason why MonoLocalBinds is a nuisance.  

As of now I think the best thing is to do (1), but define type synonyms that abbreviate the oft-repeated signatures. That should make the signatures much onerous.


| -----Original Message-----
| From: Edward Z. Yang [mailto:ezyang at MIT.EDU]
| Sent: 09 December 2010 15:28
| To: glasgow-haskell-users; Simon Peyton-Jones
| Subject: MonoLocalBinds and hoopl
| 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

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