cabal-version:  3.0
name:           if-instance
version:        0.2.1.0
synopsis:       Branch on whether a constraint is satisfied
license:        BSD-3-Clause
build-type:     Simple
author:         Sam Derbyshire
maintainer:     Sam Derbyshire
copyright:      2021 Sam Derbyshire
homepage:       https://github.com/sheaf/if-instance
category:       Type System, Plugin
description:

  This library provides a mechanism that can be used to branch on
  whether a constraint is satisfied (not limited to typeclass instances,
  despite the name of the library).

  Usage example:

  @

  {-# OPTIONS_GHC -fplugin=IfSat.Plugin #-}

  module MyModule where

  import Data.Constraint.If ( IfSat(ifSat) )

  hypot :: forall a. ( Floating a, IfSat (FMA a) ) => a -> a -> a
  hypot = ifSat @(FMA a) withFMA withoutFMA
    where
      withFMA :: FMA a => a -> a -> a
      withFMA a b =
        let
          h = sqrt $ fma a a (b * b)
          h² = h * h
          a² = a * a
          x = fma (-b) b (h² - a²) + fma h h (-h²) - fma a a (-a²)
        in
          h - x / ( 2 * h )
      withoutFMA :: a -> a -> a
      withoutFMA a b = sqrt ( a * a + b * b )
  @

  Here we select between two ways of computing the hypotenuse function
  based on whether we have access to the fused multiply-add operation

  @ fma :: FMA a => a -> a -> a -> a @

  which computes @ \\ a b c -> ( a * b ) + c @ in a single instruction,
  providing stronger guarantees about precision of the resul.

  A call of the form @hypot \@MyNumberType@ will either use the robust @withFMA@
  function when an @FMA MyNumberType@ instance is available, or will fallback
  to the simple @withoutFMA@ implementation when no such instance can be found.

extra-source-files:
  changelog.md

common common

  build-depends:
    base
      >= 4.14.0 && < 4.18,
    ghc
      >= 9.0    && < 9.6,

  default-language:
    Haskell2010

  ghc-options:
    -Wall
    -Wcompat
    -fwarn-missing-local-signatures
    -fwarn-incomplete-uni-patterns
    -fwarn-missing-deriving-strategies
    -fno-warn-unticked-promoted-constructors

library

  import:
    common

  hs-source-dirs:
    src

  build-depends:
    ghc-tcplugin-api
      >= 0.5.1.0 && < 0.6,

  exposed-modules:
    Data.Constraint.If
    IfSat.Plugin

  default-language:
    Haskell2010

  ghc-options:
    -Wall
    -Wcompat
    -fwarn-missing-local-signatures
    -fwarn-incomplete-uni-patterns
    -fwarn-missing-deriving-strategies
    -fno-warn-unticked-promoted-constructors

library if-instance-example

  import:
    common

  hs-source-dirs:
    example

  build-depends:
    if-instance

  exposed-modules:
    M2

  other-modules:
    M1

test-suite if-instance-test

  import:
    common

  type:
    exitcode-stdio-1.0

  build-depends:
    if-instance

  hs-source-dirs:
    test

  main-is:
    Main.hs