I've recently been digging around to see if there has ever been any
attempt to perform any sort of data reuse optimization in functional
languages.
Along with many other papers, I've come across `Once upon a type' as well
as `Once upon a polymorphic type'. They both mention a desire to include a
similar system in GHC.
Then in `Playing by the rules: rewriting as a practical optimization
technique in GHC' I see usage types mentioned in section 5.3:
"""
To express this, GHC adds extra usage type arguments to map, both at its
definition and at its call sites. Once this is done, a specialised version
of map can be compiled for the case when the usage-type argument is
“once”, and a rule generated to match such calls, in exactly the same way
as for specialising overloading.
"""
I can't tell if this is simply a hypothetical optimization or if this
really happened at one point in the history of GHC.
I was hoping somebody could shed some light on this topic
Was a usage type system or something similar (e.g. linear type system)
ever present in GHC?
If it was then why was it taken out (unless I'm missing something, this
optimization is not happening today)? I could guess at this and say that
the gains weren't worth the overhead, but perhaps there's another reason.
If it was never present, why not? There was obviously at least some
interest in this sort of optimization at some point in the Haskell
community and I'd be curious as to why it was never tested in GHC.
Thanks!
Philip Dexter
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