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 only derive KnownNat constraints consisting of:

i.e. it cannot derive a KnownNat (n-1) constraint from a KnownNat n constraint

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, 0.7, 0.7.1
Change log CHANGELOG.md
Dependencies base (==4.9.*), ghc (>=8.0.1 && <8.2), ghc-tcplugins-extra (>=0.2) [details]
License BSD-2-Clause
Copyright Copyright © 2016 University of Twente
Author Christiaan Baaij
Maintainer christiaan.baaij@gmail.com
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 Wed Aug 10 17:41:44 UTC 2016
Distributions Arch:0.6, LTSHaskell:0.6, NixOS:0.7.1, Stackage:0.6
Downloads 7595 total (659 in the last 30 days)
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Readme for ghc-typelits-knownnat-0.1

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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 only derive KnownNat constraints consisting of:

  • Type-level naturals
  • Type variables
  • Applications of the arithmetic expression: {+,*,^}.

i.e. it cannot derive a KnownNat (n-1) constraint from a KnownNat n constraint

To use the plugin, add the

OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver

Pragma to the header of your file.