ghc-ghc-typelits-knownnat

Derive KnownNat constraints from other KnownNat constraints

A type checker plugin for GHC that can derive "complex" 'KnownNat' constraints from other simple/variable 'KnownNat' constraints. i.e. without this plugin, you must have both a 'KnownNat n' and a 'KnownNat (n+2)' constraint in the type signature of the following function: ' f :: forall n . (KnownNat n, KnownNat (n+2)) => Proxy n -> Integer f _ = natVal (Proxy :: Proxy n) + natVal (Proxy :: Proxy (n+2)) ' Using the plugin you can omit the 'KnownNat (n+2)' constraint: ' f :: forall n . KnownNat n => Proxy n -> Integer f _ = natVal (Proxy :: Proxy n) + natVal (Proxy :: Proxy (n+2)) ' The plugin can derive 'KnownNat' constraints for types consisting of: * Type variables, when there is a corresponding 'KnownNat' constraint * Type-level naturals * Applications of the arithmetic expression: +,-,*,^ * Type functions, when there is either: 1. a matching given 'KnownNat' constraint; or 2. a corresponding 'KnownNat<N>' instance for the type function To use the plugin, add the ' OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver ' Pragma to the header of your file.

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