#12271: Remove magic from type defaulting -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The Prelude is great, but there are reasons people use drop-in replacements as well. In particular, I'm using [numeric-prelude] for its more mathematical numeric type class hierarchy. Unfortunately, I don't get type defaulting with their numeric type class replacements, which makes some operations a real pain to use. For example, GHC accepts the following: {{{ main :: IO () main = print $ 2^2 }}} but not {{{ {-# LANGUAGE NoImplicitPrelude, RebindableSyntax #-} import Algebra.Ring as Ring (C) import Algebra.ToInteger as ToInteger (C) import NumericPrelude hiding ((^)) (^) :: (Ring.C a, ToInteger.C i) => a -> i -> a x0 ^ y0 = undefined main :: IO () main = print $ 2^2 }}} (there are ambiguous types on both occurrences of `2`.) The [https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-790004.... HaskellReport] seems to suggest that there is no way to use the `default` keyword to get type defaulting in this case. What I'm asking for is a programmatic way to set defaulting rules for non- standard numeric classes. In terms of implementation, my naive approach would be to remove `Num` from the process altogether: if we ever see a "integer-looking" literal and its type cannot be inferred, always default it to the `IntDefault` type (which could be the same as the current "automatic" default, or the value in the `default` tuple if present). I'm guessing that this approach has some flaws; perhaps someone can point them out? Barring that, I suppose we need a way to specify a `Num` replacement. In the case of numeric-prelude, it would be `Algebra.Additive.C`. An even more flexible approach would be to allow defaulting for constants with a specific constraint. (That's equivalent to specifying a default type for constants with the constraint `Num a => a` and a possibly different default for `Integral a => a`.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12271 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler