#13001: EnumFromThenTo is is not a good producer -------------------------------------+------------------------------------- Reporter: George | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: MacOS X Architecture: x86_64 | Type of failure: Runtime (amd64) | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs {-# OPTIONS_GHC -Wall #-} module Foo where testFromTo :: Int -> Int testFromTo n = length ([0..(10^n)] :: [Int]) testFromThenTo :: Int -> Int testFromThenTo n = length ([0,2..(10^n)] :: [Int]) }}} {{{ $ ghci -fobject-code -O GHCi, version 8.0.1: http://www.haskell.org/ghc/ :? for help Prelude> :load Foo :load Foo [1 of 1] Compiling Foo ( Foo.hs, Foo.o ) Ok, modules loaded: Foo (Foo.o). Prelude Foo> :set +s :set +s Prelude Foo> testFromTo 5 testFromTo 5 100001 (0.02 secs, 97,992 bytes) Prelude Foo> testFromThenTo 5 testFromThenTo 5 50001 (0.00 secs, 5,694,424 bytes) Prelude Foo> testFromThenTo 6 testFromThenTo 6 500001 (0.02 secs, 56,095,288 bytes) Prelude Foo> }}} I set the Type to bug rather than feature request as the source code in http://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.Enum.html#enumF... seems to be trying to do list fusion: {{{#!hs -- efdInt and efdtInt deal with [a,b..] and [a,b..c]. -- The code is more complicated because of worries about Int overflow. -- See Note [How the Enum rules work] {-# RULES "efdtInt" [~1] forall x1 x2 y. efdtInt x1 x2 y = build (\ c n -> efdtIntFB c n x1 x2 y) "efdtIntUpList" [1] efdtIntFB (:) [] = efdtInt #-} }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13001 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler