Normalise GHC.TypeLits.Nat equations
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README.md

ghc-typelits-natnormalise

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A type checker plugin for GHC that can solve equalities of types of kind Nat, where these types are either:

  • Type-level naturals
  • Type variables
  • Applications of the arithmetic expressions (+,-,*,^).

It solves these equalities by normalising them to sort-of SOP (Sum-of-Products) form, and then perform a simple syntactic equality.

For example, this solver can prove the equality between:

(x + 2)^(y + 2)

and

4*x*(2 + x)^y + 4*(2 + x)^y + (2 + x)^y*x^2

Because the latter is actually the SOP normal form of the former.

To use the plugin, add

{-# OPTIONS_GHC -fplugin GHC.TypeLits.Normalise #-}

To the header of your file.