I suspect the issue here is that `(==)` is being inlined too early, hiding the rule fire opportunity. By contrast, if you give `(==)` another name with a later inline pragma the `break -> breakByte` rule fires as expected.
{{{#!hs main = do chunk <- DB.hGetSome stdin 32768 let newline = c2w '\n' let (prefix,suffix) = DB.break (eq newline) chunk DB.hPut stdout prefix DB.hPut stdout suffix
eq :: Eq a => a -> a -> Bool eq = (==) {-# INLINE [1] eq #-}
{-# RULES "ByteString specialise break (eq x)" forall x. DB.break (eq x) = DB.breakByte x #-} }}}
Actually, even if one doesn't specify an explicit phase on `eq` the rule still fires! This is likely accidental, since GHC just happens to look at
#11688: Bytestring break failing rewrite to breakByte and failing to eliminate boxing/unboxing -------------------------------------+------------------------------------- Reporter: alexbiehl | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by bgamari): I briefly looked at this; I left a comment on the `bytestring` ticket which I'll reproduce here, the `breakByte` rule before the `eq` inlining. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11688#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler