#12364: Demand analysis for sum types -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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 jmct): Possibly, but GHC's much better about detecting recursiveness for product types, see the large section titled "Deciding which type constructors are recursive" in compiler/typecheck/TcTyDecls.hs. In particular there's this section: {{{ The "recursive" flag for algebraic data types is irrelevant (never consulted) for types with more than one constructor. An algebraic data type M.T is "recursive" iff it has just one constructor, and (a) it is declared in an hi-boot file (see RdrHsSyn.hsIfaceDecl) (b) it is declared in a source file, but that source file has a companion hi-boot file which declares the type or (c) one can get from its arg types to T via type synonyms, or by non-recursive newtypes or non-recursive product types in M e.g. data T = MkT (T -> Int) Bool }}} I'm going to speculate wildly and say that it just hasn't been that important for GHC to detect recursiveness in sum types in the past, and therefore it just hasn't gotten very much attention. There probably shouldn't be a difference in theory, but GHC as it is today is better at avoiding recursiveness in product types and therefore would be better at avoiding the issue in that case. Though you're right in principle. Tomorrow morning I'll dig up my old notes and comment on #12368 giving an example. I don't want to confuse the two issues too much, though they're intimately related. #12368 is really about recursion in the _functions_ though, which is an important distinction (and I definitely have examples where the current behavior of the 'cunning plan' is necessary). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12364#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler