#9107: An alternative to witness values ----------------------------------------------+---------------------------- Reporter: spacekitteh | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Moderate (less than a day) | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: ----------------------------------------------+---------------------------- So, I might have come up with a nicer notation to providing type witnesses. Say I have a class thusly: {{{ class Metric a where measure :: Vector f -> Vector f -> f }}} and an example instance: {{{ instance Metric "Euclidean" where measure = sum . (zipWith (*)) }}} In order to actually use a specific metric, one would have to modify the type signature to include a witness, and use it such as {{{measure undefined::"Euclidean" a b}}}. Personally, I find such expressions to be ugly clutter. Instead, here are three alternate proposals which don't require including a witness: 1. {{{(Metric "Euclidean") => <term involving measure>}}} for example {{{dot a b = (Metric "Euclidean") => measure a b}}} 2. New keyword similar to "where" {{{ dot a b = measure a b assuming (Metric "Euclidean") }}} 3. New keyword similar to "let..in" {{{dot a b = assume (Metric "Euclidean") in measure a b}}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9107 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler