Package Info


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.

License: BSD-2-Clause



Package Version Update ID Released Package Hub Version Platforms Subpackages
0.3-bp150.1.3 info GA Release 2018-07-30 15
  • AArch64
  • ppc64le
  • x86-64
  • ghc-ghc-typelits-knownnat
  • ghc-ghc-typelits-knownnat-devel