Is this a GHC bug?

Question

I have the following program and it will not compile; it just gets stuck in Compiling Main. I'm not sure why it freezes, because it is either a mistake, or am I missing something?

import qualified Data.Vector.Unboxed as V test :: [Int] test = V.foldl' unique [] k where n = 10000000 k = V.generate n (\i -> i `div` 1000 * 1000) unique [] k = [k] unique acc@ (ck:_) k | k == ck = acc | otherwise = k:acc main :: IO () main = print $ head test 

ghc --make -O3 -Odph -fllvm test.hs

Answer 1

Yes, I think this is a mistake: https://ghc.haskell.org/trac/ghc/ticket/5550

Using -v , you will see that the GHC hangs while optimizing SpecConstr :

 ghc-7.6.3 test.hs -O2 -fforce-recomp -v Glasgow Haskell Compiler, Version 7.6.3, stage 2 booted by GHC version 7.0.4 Using binary package database: /nix/store/x5rdp4smhc6q3rxbw4x1akj10z3h0lvg-haskell-env-ghc-7.6.3/lib/ghc-7.6.3/package.conf.d/package.cache hiding package Cabal-1.16.0 to avoid conflict with later version Cabal-1.18.1.3 hiding package QuickCheck-2.6 to avoid conflict with later version QuickCheck-2.7.3 hiding package binary-0.5.1.1 to avoid conflict with later version binary-0.7.1.0 wired-in package ghc-prim mapped to ghc-prim-0.3.0.0-d5221a8c8a269b66ab9a07bdc23317dd wired-in package integer-gmp mapped to integer-gmp-0.5.0.0-2f15426f5b53fe4c6490832f9b20d8d7 wired-in package base mapped to base-4.6.0.1-02ac91ff7de681afee6a8fa62f0c87f3 wired-in package rts mapped to builtin_rts wired-in package template-haskell mapped to template-haskell-2.8.0.0-0cee70fbb279a3d93dcc4ec502790a2b wired-in package dph-seq not found. wired-in package dph-par not found. Hsc static flags: -static *** Chasing dependencies: Chasing modules from: *test.hs Stable obj: [] Stable BCO: [] Ready for upsweep [NONREC ModSummary { ms_hs_date = 2014-04-15 18:43:29 UTC ms_mod = main:Main, ms_textual_imps = [import (implicit) Prelude, import qualified Data.Vector.Unboxed as V] ms_srcimps = [] }] *** Deleting temp files: Deleting: compile: input file test.hs Created temporary directory: /tmp/ghc23870_0 *** Checking old interface for main:Main: [1 of 1] Compiling Main ( test.hs, test.o ) *** Parser: *** Renamer/typechecker: *** Desugar: Result size of Desugar (after optimization) = {terms: 55, types: 44, coercions: 0} *** Simplifier: Result size of Simplifier iteration=1 = {terms: 46, types: 34, coercions: 5} Result size of Simplifier iteration=2 = {terms: 48, types: 40, coercions: 5} Result size of Simplifier = {terms: 48, types: 40, coercions: 5} *** Specialise: Result size of Specialise = {terms: 48, types: 40, coercions: 5} *** Float out(FOS {Lam = Just 0, Consts = True, PAPs = False}): Result size of Float out(FOS {Lam = Just 0, Consts = True, PAPs = False}) = {terms: 66, types: 58, coercions: 5} *** Float inwards: Result size of Float inwards = {terms: 66, types: 58, coercions: 5} *** Simplifier: Result size of Simplifier iteration=1 = {terms: 76, types: 66, coercions: 36} Result size of Simplifier iteration=2 = {terms: 71, types: 60, coercions: 26} Result size of Simplifier = {terms: 71, types: 60, coercions: 26} *** Simplifier: Result size of Simplifier iteration=1 = {terms: 143, types: 113, coercions: 33} Result size of Simplifier iteration=2 = {terms: 99, types: 79, coercions: 30} Result size of Simplifier = {terms: 99, types: 79, coercions: 30} *** Simplifier: Result size of Simplifier iteration=1 = {terms: 120, types: 88, coercions: 26} Result size of Simplifier iteration=2 = {terms: 90, types: 64, coercions: 16} Result size of Simplifier iteration=3 = {terms: 88, types: 63, coercions: 16} Result size of Simplifier = {terms: 88, types: 63, coercions: 16} *** Demand analysis: Result size of Demand analysis = {terms: 88, types: 63, coercions: 16} *** Worker Wrapper binds: Result size of Worker Wrapper binds = {terms: 108, types: 80, coercions: 23} *** Simplifier: Result size of Simplifier iteration=1 = {terms: 100, types: 72, coercions: 13} Result size of Simplifier iteration=2 = {terms: 91, types: 58, coercions: 9} Result size of Simplifier = {terms: 91, types: 58, coercions: 9} *** Float out(FOS {Lam = Just 0, Consts = True, PAPs = True}): Result size of Float out(FOS {Lam = Just 0, Consts = True, PAPs = True}) = {terms: 93, types: 59, coercions: 9} *** Common sub-expression: Result size of Common sub-expression = {terms: 93, types: 59, coercions: 9} *** Float inwards: Result size of Float inwards = {terms: 93, types: 59, coercions: 9} *** Liberate case: Result size of Liberate case = {terms: 93, types: 59, coercions: 9} *** Simplifier: Result size of Simplifier iteration=1 = {terms: 93, types: 59, coercions: 9} Result size of Simplifier = {terms: 93, types: 59, coercions: 9} *** SpecConstr: Result size of SpecConstr 

You can also verify that with -fno-spec-constr or -fno-enable-rewrite-rules error will disappear. He tested it with the latest GHC (7.8.2) and the problem seems to be fixed there.