attenuation (empty) → 0.1.0.0

raw patch · 4 files changed

+498/−0 lines, 4 filesdep +basedep +profunctors

Dependencies added: base, profunctors

Files

• CHANGELOG.md +4/−0
• LICENSE +202/−0
• attenuation.cabal +42/−0
• src/Data/Type/Attenuation.hs +250/−0

@@ -0,0 +1,4 @@+# 0.1.0.0++Added everything that exists in this version.  See Haddock documentation for a+complete accounting.

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@@ -0,0 +1,42 @@+cabal-version: 1.12++-- This file has been generated from package.yaml by hpack version 0.33.0.+--+-- see: https://github.com/sol/hpack+--+-- hash: 92d47a2950ec019ac542be7970e219dab7309ba103c1c46acae9beb21fef6f06++name:           attenuation+version:        0.1.0.0+synopsis:       Representational subtyping relations and variance roles.+description:    This lets you coerce containers (among other things) from stronger types to+                weaker types with zero runtime cost when it's safe to do so, e.g.+                @[Fin n] -> [Int]@.  This primarily comes into play when using newtypes to+                impose additional invariants on existing types.+category:       Constraints+homepage:       https://github.com/google/hs-attenuation#readme+bug-reports:    https://github.com/google/hs-attenuation/issues+author:         Andrew Pritchard <awpr@google.com>+maintainer:     Andrew Pritchard <awpr@google.com>+copyright:      2020-2021 Google LLC+license:        Apache-2.0+license-file:   LICENSE+build-type:     Simple+extra-source-files:+    CHANGELOG.md++source-repository head+  type: git+  location: https://github.com/google/hs-attenuation++library+  exposed-modules:+      Data.Type.Attenuation+  other-modules:+      Paths_attenuation+  hs-source-dirs:+      src+  build-depends:+      base >=4.7 && <4.16+    , profunctors >=0.1 && <5.7+  default-language: Haskell2010

@@ -0,0 +1,250 @@+-- Copyright 2020-2021 Google LLC+--+-- Licensed under the Apache License, Version 2.0 (the "License");+-- you may not use this file except in compliance with the License.+-- You may obtain a copy of the License at+--+--      http://www.apache.org/licenses/LICENSE-2.0+--+-- Unless required by applicable law or agreed to in writing, software+-- distributed under the License is distributed on an "AS IS" BASIS,+-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.+-- See the License for the specific language governing permissions and+-- limitations under the License.++-- | Representational subtyping relations and variance roles.++{-# LANGUAGE ConstraintKinds #-}+{-# LANGUAGE CPP #-}+{-# LANGUAGE DataKinds #-}+{-# LANGUAGE GADTs #-}+{-# LANGUAGE QuantifiedConstraints #-}+{-# LANGUAGE RankNTypes #-}+{-# LANGUAGE ScopedTypeVariables #-}+{-# LANGUAGE TypeFamilies #-}+{-# LANGUAGE TypeOperators #-}++module Data.Type.Attenuation+         ( -- * Attenuation+           Attenuation, attenuate, coercible+         , trans, repr, coer+           -- ** Representationality+         , Representational, Representational0, Representational1, Variance+           -- ** Functor and Contravariant+         , co, contra+           -- ** Bifunctor+         , fstco, sndco+           -- ** Profunctor+         , lcontra, rco+           -- ** Initial and Final Objects+         , attVoid, attAny+         ) where++import Control.Category (Category(..))+import Data.Bifunctor (Bifunctor)+import Data.Coerce (Coercible, coerce)+import Data.Functor.Contravariant (Contravariant)+import Data.Kind (Constraint)+import Data.Profunctor (Profunctor)+import Data.Type.Coercion (Coercion(..), sym)+import qualified Data.Type.Coercion as Coercion+import Data.Type.Equality ((:~:)(..))+import Data.Void (Void)+import GHC.Exts (Any)++#if MIN_VERSION_base(4,15,0)+import Unsafe.Coerce (unsafeEqualityProof, UnsafeEquality(..))+#else+import Unsafe.Coerce (unsafeCoerce)+#endif++-- | @Attenuation a b@ is a unidirectional 'Coercion' from @a@ to @b@.+--+-- "Attenuate: reduce the force, effect, or value of."  An @Attenuation@ takes+-- a stronger, stricter type to a weaker, more lax type.  It's meant to sound a+-- bit like 'Coercion', while conveying that it weakens the type of its+-- argument.+--+-- This arises from newtypes that impose additional invariants on their+-- representations: if we define @Fin :: Nat -> Type@ as a newtype over 'Int',+-- such as in <https://hackage.haskell.org/package/fin-int fin-int>, then it's+-- safe to 'coerce' @Fin@s to 'Int's, and @Fin@s to other @Fin@s with larger+-- @Nat@ parameters, but not vice versa.+--+-- Within the module defining this @Fin@ type, we can obtain 'Coercible'+-- between any two @Fin@ types regardless of their roles, because their newtype+-- constructors are in scope, but if we've taken appropriate precautions+-- (namely not exporting the constructor), we can't obtain it outside the+-- module.  We can relax this and make the coercion "opt-in" by exporting it in+-- the form of a 'Coercion' with a scary name like @unsafeCoFin@, but this is+-- still error-prone.+--+-- Instead, we introduce a newtype wrapper around 'Coercion' which restricts it+-- to be used only in the forward direction, and carefully design its API so+-- that it can only be obtained under the appropriate circumstances.+--+-- @Attenuation a b@ can be seen as a witness that @a@ is, semantically and+-- representationally, a subtype of @b@: that is, any runtime object that+-- inhabits @a@ also inhabits @b@ without any conversion.  Note, however, that+-- we can't have a useful typeclass of this subtyping relation because all of+-- its instances would have to be specified individually: whereas 'Coercible'+-- is willing to invert or compose coercions implicitly because of GHC magic, a+-- subtyping class would not have that affordance.+newtype Attenuation a b = Attenuation (Coercion a b)+  deriving (Eq, Ord, Show)++instance Category Attenuation where+  id = coercible+  (.) = flip trans++-- | Coerce along an 'Attenuation'.+--+-- This is really, truly a coercion when it reaches Core.+attenuate :: Attenuation a b -> a -> b+attenuate (Attenuation Coercion) = coerce++-- | Any coercible types have an 'Attenuation'.+coercible :: Coercible a b => Attenuation a b+coercible = Attenuation Coercion++-- | Transitivity of 'Attenuation's.  See also the 'Category' instance.+trans :: Attenuation a b -> Attenuation b c -> Attenuation a c+trans (Attenuation coAB) (Attenuation coBC) =+  Attenuation (Coercion.trans coAB coBC)++-- | Any type is unidirectionally coercible to itself.+repr :: (a :~: b) -> Attenuation a b+repr Refl = Attenuation Coercion++-- | Bidirectional coercions can be weakened to unidirectional coercions.+coer :: Coercion a b -> Attenuation a b+coer = Attenuation++-- | A witness that @a@ occurs representationally in @s@ and that, when+-- substituting it for @b@, you get @t@.+--+-- These compose like Lenses from the "lens" package, so you can e.g. lift+-- 'Attenuation's through several @Functor@s by @'co'.co.co $ x@.+type Variance s t a b = Attenuation a b -> Attenuation s t++-- | A constraint that behaves like @type role f representational@.+--+-- This means that if we have this constraint in context and GHC can solve+-- 'Coercible' for some types @a@ and @b@, it will also lift the coercion to @f+-- a@ and @f b@.+type Representational f =+  (forall a b. Coercible a b => Coercible (f a) (f b) :: Constraint)++-- | Lift a 'Coercion' over a type constructor.+rep :: Representational f => Coercion a b -> Coercion (f a) (f b)+rep Coercion = Coercion++-- | A constraint that behaves like @type role f _ representational@.+--+-- See also 'Representational'.+type Representational1 f =+  (forall a b x. Coercible a b => Coercible (f x a) (f x b) :: Constraint)++-- | A constraint that behaves like @type role f representational _@.+--+-- See also 'Representational'.+type Representational0 f =+  (forall a b x. Coercible a b => Coercible (f a x) (f b x) :: Constraint)++-- | Lift a 'Coercion' over the last-but-one parameter of a type constructor.+rep0 :: Representational0 f => Coercion a b -> Coercion (f a x) (f b x)+rep0 Coercion = Coercion++-- | Lift an 'Attenuation' covariantly over a type constructor @f@.+--+-- Although we don't /use/ the 'Functor' constraint, it serves an important+-- purpose: to guarantee that the type parameter @a@ doesn't appear+-- contravariantly in @f a@; otherwise it'd be impossible to write a 'Functor'+-- instance.  This is used as a standin for more-detailed "covariant" and+-- "contravariant" type roles, which GHC doesn't have because there's no+-- built-in notion of subtyping to use them with.  'Representational1' provides+-- the actual lifting of coercions, and 'Functor' guarantees we've got the+-- variance right.+co :: (Functor f, Representational f) => Variance (f a) (f b) a b+co (Attenuation c) = Attenuation (rep c)++-- | Lift an 'Attenuation' contravariantly over a type constructor @f@.+--+-- Regarding the 'Contravariant' constraint, see 'co', and interchange mentions+-- of covariance and contravariance.+contra :: (Contravariant f, Representational f) => Variance (f b) (f a) a b+contra (Attenuation c) = Attenuation (sym $ rep c)++-- | Lift an 'Attenuation' covariantly over the left of a 'Bifunctor'.+--+-- Like with 'co' and 'contra', we require a not-actually-used constraint as+-- proof that the type has the appropriate variance.  Since there's not a+-- commonly-used class for functors over the last-but-one parameter, we use+-- 'Bifunctor'.  Sadly, this rules out types which are covariant in parameter+-- -1 and contravariant in parameter -0.+fstco :: (Bifunctor f, Representational0 f) => Variance (f a x) (f b x) a b+fstco (Attenuation c) = Attenuation (rep0 c)++-- | Lift an 'Attenuation' covariantly over the last-but-one type parameter.+--+-- Like with 'co' and 'contra', we require a not-actually-used constraint as+-- proof that the type has the appropriate variance.  Since there's not a+-- commonly-used class for functors over the last-but-one parameter, we use+-- 'Bifunctor'.  Sadly, this rules out types which are covariant in parameter+-- -1 and contravariant in parameter -0.+--+-- Note that any particular type with a @Bifunctor f@ instance should also have+-- @Functor (f x)@, so 'co' should work on any type that 'sndco' works on, but+-- in polymorphic contexts, the 'Functor' instance may not be available.+sndco :: (Bifunctor f, Representational1 f) => Variance (f x a) (f x b) a b+sndco (Attenuation c) = Attenuation (rep c)++-- | Lift an 'Attenuation' covariantly over the left of a 'Profunctor'.+--+-- Similarly to the use of 'Functor' in 'co', we use 'Profunctor' to guarantee+-- contravariance in the appropriate parameter.+lcontra :: (Profunctor p, Representational0 p) => Variance (p b x) (p a x) a b+lcontra (Attenuation c) = Attenuation (sym $ rep0 c)++-- | Lift an 'Attenuation' covariantly over the right of a 'Profunctor'.+--+-- Similarly to the use of 'Functor' in 'co', we use 'Profunctor' to guarantee+-- contravariance in the appropriate parameter.+--+-- As with 'sndco', this functions the same as 'co', but the needed 'Functor'+-- instance might not be available in polymorphic contexts.+rco :: (Profunctor p, Representational1 p) => Variance (p x a) (p x b) a b+rco (Attenuation c) = Attenuation (rep c)++-- | 'Attenuation' of 'Void' to any type.+--+-- If you have 'Void' appearing covariantly in a type, you can replace it with+-- any other lifted type with a coercion, because the value can't contain any+-- non-bottom 'Void' values (there are none), and any value that /is/ bottom+-- can "work" (i.e. throw or fail to terminate) just as well at any other+-- lifted type.+--+-- For example, if you have a @[Doc Void]@ (from+-- <https://hackage.haskell.org/package/pretty pretty>), you know it doesn't+-- have any annotations (or they're errors), so you can use it as @[Doc a]@+-- without actually traversing the list and @Doc@ structure to apply+-- 'Data.Void.absurd' to all of the 'Void's.+attVoid :: forall a. Attenuation Void a+attVoid = Attenuation $+#if MIN_VERSION_base(4,15,0)+  case unsafeEqualityProof :: UnsafeEquality a Void of UnsafeRefl -> Coercion+#else+  (unsafeCoerce (Coercion :: Coercion a a) :: Coercion Void a)+#endif++-- | 'Attenuation' of any type to 'Any'.+--+-- Similarly to 'attVoid', you can weaken any type to 'Any' for free, since any+-- value is a valid value of type 'Any'.+attAny :: forall a. Attenuation a Any+attAny = Attenuation $+#if MIN_VERSION_base(4,15,0)+  case unsafeEqualityProof :: UnsafeEquality a Any of UnsafeRefl -> Coercion+#else+  (unsafeCoerce (Coercion :: Coercion a a) :: Coercion a Any)+#endif