Type (class) recursion + families = exponential compile time?

Ben Franksen ben.franksen at online.de
Thu Feb 26 18:12:07 EST 2009


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:

ben at sarun[1] > time ghc -c Bug2.hs
ghc -c Bug2.hs  1,79s user 0,04s system 99% cpu 1,836 total

ben at sarun[1] > time ghc -c Bug2.hs
ghc -c Bug2.hs  5,87s user 0,14s system 99% cpu 6,028 total

ben at sarun[1] > time ghc -c Bug2.hs
ghc -c Bug2.hs  23,52s user 0,36s system 99% cpu 23,899 total

ben at 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.

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