ghc-typelits-knownnat: Derive KnownNat constraints from other KnownNat constraints

[ bsd2, library, type-system ] [ Propose Tags ]

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:

  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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Versions [faq] 0.1, 0.1.1, 0.2, 0.2.1, 0.2.2, 0.2.3, 0.2.4, 0.3, 0.3.1, 0.4, 0.4.1, 0.4.2, 0.5, 0.5.1, 0.6
Change log CHANGELOG.md
Dependencies base (>=4.9 && <5), ghc (>=8.0.1 && <8.6), ghc-tcplugins-extra (>=0.2.5 && <0.3), ghc-typelits-natnormalise (>=0.5.10 && <0.6), template-haskell (>=2.11.0.0 && <2.14), transformers (>=0.5.2.0 && <0.6) [details]
License BSD-2-Clause
Copyright Copyright © 2016 , University of Twente, 2017-2018, QBayLogic B.V., 2017 , Google Inc.
Author Christiaan Baaij
Maintainer christiaan.baaij@gmail.com
Revised Revision 1 made by ChristiaanBaaij at Fri May 11 20:43:04 UTC 2018
Category Type System
Home page http://clash-lang.org/
Source repo head: git clone https://github.com/clash-lang/ghc-typelits-knownnat.git
Uploaded by ChristiaanBaaij at Sun Apr 15 08:56:35 UTC 2018
Distributions Arch:0.6, LTSHaskell:0.6, NixOS:0.6, Stackage:0.6
Downloads 3955 total (255 in the last 30 days)
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ghc-typelits-knownnat

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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:
    • a matching given KnownNat constraint; or
    • a corresponding KnownNat<N> instance for the type function

To elaborate the latter points, given the type family Min:

type family Min (a :: Nat) (b :: Nat) :: Nat where
  Min 0 b = 0
  Min a b = If (a <=? b) a b

the plugin can derive a KnownNat (Min x y + 1) constraint given only a KnownNat (Min x y) constraint:

g :: forall x y . (KnownNat (Min x y)) => Proxy x -> Proxy y -> Integer
g _ _ = natVal (Proxy :: Proxy (Min x y + 1))

And, given the type family Max:

type family Max (a :: Nat) (b :: Nat) :: Nat where
  Max 0 b = b
  Max a b = If (a <=? b) b a

and corresponding KnownNat2 instance:

instance (KnownNat a, KnownNat b) => KnownNat2 "TestFunctions.Max" a b where
  natSing2 = let x = natVal (Proxy @ a)
                 y = natVal (Proxy @ b)
                 z = max x y
             in  SNatKn z
  {-# INLINE natSing2 #-}

the plugin can derive a KnownNat (Max x y + 1) constraint given only a KnownNat x and KnownNat y constraint:

h :: forall x y . (KnownNat x, KnownNat y) => Proxy x -> Proxy y -> Integer
h _ _ = natVal (Proxy :: Proxy (Max x y + 1))

To use the plugin, add the

OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver

Pragma to the header of your file.