> Using this combinator instead of writing the algorithm directly cost me 30%
> allocation overhead!
What does your algorithm look like when you write it directly? Something like
this:
flatten_many fmode roles tys
= unzip `liftM` mapM go (zip roles tys)
where
go (Nominal,ty) = flatten_one (fmode { fe_eq_rel = NomEq }) ty
go (Representational,ty) = flatten_one (fmode { fe_eq_rel = ReprEq }) ty
go (Phantom, ty) = -- See Note [Phantoms in the flattener]
return (ty, mkTcPhantomCo ty ty)
?
Maybe this has something to do with `zipWithAndUnzipM` not being tail-recursive
vs. direct version
being able to fuse intermediate lists?
Janek
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