Hi, The following reduced test-case:
{-# LANGUAGE DataKinds, TypeOperators, TypeFamilies, ConstraintKinds, FlexibleContexts, BangPatterns #-} module ConstraintLeak where
import GHC.TypeLits import Data.Proxy
type Constraint1 n = ( Constraint2 n , Constraint2 (2^n) ) type Constraint2 n = ( KnownNat n , ((n-1)+1) ~ n -- Comment this line to run in constant space )
test :: Constraint1 x => Proxy x -> String test !s | s == s = test s
main :: IO () main = putStr (test (Proxy :: Proxy 3))
contains a space-leak when compiled without optimisations (i.e. -O0 or in GHCi). It's true that the code doesn't actually do anything (when compiled with -O you get "Exception <<loop>>"), but I'm using it to exemplify some code that runs for some time, and then prints something. As you can see, 'test' is strict in its arguments, but it seems lazy in its constraints. When I look at the heap profile (-hy), I see it rapidly accumulating values of the following types: - ~ - KnownNat - (,) The problem with these constraints, unlike a normal type-class constraints, is that I cannot 'seq' one of its members to force the dictionary and hence be strict in the constraints. We stumbled upon this particular space-leak while trying to find a space-leak in our own code; we don't think it's the real culprit in our own code, but find it disturbing nonetheless. GHCi is heavily used in our work-flow, meaning we often run in an -O0 setting, so I would like to know if it is possible to plug this constraint-induced space-leak. Regards, Christiaan Baaij