#11518: Test TcCoercibleFail hangs with substitution sanity checks enabled -------------------------------------+------------------------------------- Reporter: niteria | Owner: niteria Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by niteria): If you unwrap it 100 times you end up with a type that has size `2^100`. Computing free vars of such type would take forever. And the ASSERT in `substTy` does exactly that. It's enough to unwrap it 30 times for it to become unbearably slow. That type is very regular and a very compact representation because of sharing. The type is essentially: {{{ a0 = () a1 = (a0, a0) -- note the sharing, but when free vars are -- computed they are computed for fst and snd independently a2 = (a1, a1) a3 = (a2, a2) a100 = (a99, a99) -- this has size 2^100, O(2^n) when naively explored, -- but it's represented as a sequence of O(n) pointers. }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11518#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler