Defer instance lookups until runtime.

This plugin allows you to write

{-# OPTIONS_GHC -fplugin Data.Constraint.Emerge.Plugin #-}
module Test where

import Data.Constraint.Emerge

showAnything :: forall c. Emerge (Show c) => c -> String
showAnything c =
case emerge @(Show c) of
Just Dict -> show c
Nothing   -> "{{unshowable}}"

where the 'Emerge (Show c)' will automatically be discharged for any monomorphic c.

See test/EmergeSpec.hs for a few examples of what this plugin can do for you.

constraints-emerge: defer instance lookups until runtime

| Hackage

Dedication

Failure should be our teacher, not our undertaker. Failure is delay, not defeat. It is a temporary detour, not a dead end. Failure is something we can avoid only by saying nothing, doing nothing, and being nothing.

Denis Waitley

Synopsis

{-# LANGUAGE ConstraintKinds     #-}
{-# LANGUAGE FlexibleContexts    #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications    #-}
{-# OPTIONS_GHC -fplugin=Data.Constraint.Emerge.Plugin #-}

module Test where

import Data.Constraint.Emerge

showAnything :: forall c. Emerge (Show c) => c -> String
showAnything c =
  case emerge @(Show c) of
    Just Dict -> show c
    Nothing -> "<<unshowable>>"


showBool = showAnything True  -- "True"
showId   = showAnything id    -- "<<unshowable>>"

Known Bugs

• constraints-emerge will generate type-equality dictionaries any types (even ones that aren't equal :scream: :scream: :scream:)
• It fails to provide Emerge c dictionaries at runtime.
• The generated error messages mention mangled type variables; it would be cool if they didn't.

If someone wants to pick it up from here, that’d be great!

Related Work

• union-constraints
• Data.Constraint.Deferrable

Contact

Please reports bugs and missing features at the GitHub bugtracker. This is also where you can find the source code.

constraints-emerge was written by Sandy Maguire and is licensed under a permissive MIT license.