Here's the reference
http://portal.acm.org/citation.cfm?id=96748
"Deciding ML typability is complete for deterministic exponential
time" -- Harry G. Mairson.
Ben.
On 27/02/2009, at 10:12 AM, Ben Franksen wrote:
Hi
the attached module is a much reduced version of some type-level
assurance
stuff (inspired by the Lightweight Monadic Regions paper) I am
trying to
do. I am almost certain that it could be reduced further but it is
late and
I want to get this off my desk.
Note the 4 test functions, test11 .. test14. The following are
timings for
compiling the module only with all test functions commented out,
except
respectively, test11, test12, test13, and test14:
b...@sarun[1] > time ghc -c Bug2.hs
ghc -c Bug2.hs 1,79s user 0,04s system 99% cpu 1,836 total
b...@sarun[1] > time ghc -c Bug2.hs
ghc -c Bug2.hs 5,87s user 0,14s system 99% cpu 6,028 total
b...@sarun[1] > time ghc -c Bug2.hs
ghc -c Bug2.hs 23,52s user 0,36s system 99% cpu 23,899 total
b...@sarun[1] > time ghc -c Bug2.hs
ghc -c Bug2.hs 102,20s user 1,32s system 97% cpu 1:45,89 total
It seems something is scaling very badly. You really don't want to
wait for
a version with 20 levels of nesting to compile...
If anyone has a good explanation for this, I'd be grateful.
BTW, I am not at all certain that this is ghc's fault, it may well
be my
program, i.e. the constraints are too complex, whatever. I have no
idea how
hard it is for the compiler to do all the unification. Also, the
problem is
not of much practical relevance, as no sensible program will use
more than
a handfull levels of nesting.
Cheers
Ben
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