#16070: Better inferred signatures -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: TypeFamilies 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 simonpj): Yes, I was thinking of the following algorithm. The algorithm is used when inferring the type of a function (and only on that occasion). Specifically, it is an enhancement to the existing function `TcType.mkMinimalBySCs`. Suppose `C` is declared thus, with a type variable on the left of a superclass equality: {{{ class (b ~ ty, ...) => C a b c }}} `mkMinimalBySCs` is given a list of predicates, `preds`, such as `(Ord a, Eq a)`, or `(HasField "foo" r (FieldType "foo" r))`. It's business is to abbreviate it to an equivalent one. * For each predicate `(C t1 t2 t3)`, where `t2` = `ty[t1/a,t3/c]`, then replace all occurrences of `t2` in `preds` with `x` where is a fresh skolem variable. So if {{{ preds = [ HasField "bar" r (FieldType "bar" r) , HasField "foo" r (FieldType "bar" r) ] }}} we'll spot the condition on the first predicate, and replace `(FieldType "bar" r)` with `x` throughout: {{{ preds = [ HasField "bar" r x , HasField "foo" r x ] }}} Now we need to * Quantify over `x` as well as whatever else we were quantifying over before * Apply that same substitution to the type we are quantifying over (to catch the case of `g` in the `Description`. So it's more than just modifyign `mkMinimalBySCs`. There's something a bit unsatisfying about this: during flattening we have all these common sub-expressions neatly identified. It seems a pity to un- flatten them away only to then have to rediscover them. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16070#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler