constraints-emerge: defer instance lookups until runtime

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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:)

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.

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

Contact

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