constraints
Constraint manipulation
http://github.com/ekmett/constraints/
Version on this page:  0.13 
LTS Haskell 21.22:  0.13.4@rev:1 
Stackage Nightly 20231129:  0.13.4@rev:1 
Latest on Hackage:  0.14 
constraints0.13@sha256:d341eb4adbf712f928706928d23a173fb3d0976f0dfaf6a274958975d5fc9e75,2338
Module documentation for 0.13
Changes
0.13 [2021.02.17]

Data.Constraint.Symbol
now reexports theGHC.TypeLits.AppendSymbol
type family from recent versions ofbase
(or, on old versions ofbase
, it defines a backwardscompatibile version ofAppendSymbol
). The existing(++)
type family forData.Constraint.Symbol
is now a synonym forAppendSymbol
.This is technically a breaking change, as
(++)
was previously defined like so:type family (++) :: Symbol > Symbol > Symbol
This meant that
(++)
could be partially applied. However, for compatibility with the way thatAppendSymbol
is defined,(++)
is now defined like so:type m ++ n = AppendSymbol m n
As a result,
(++)
can no longer be partially applied. 
Make the
(++)
type family inData.Constraint.Symbol
beinfixr 5
. 
Add
implied :: (a => b) > (a : b)
toData.Constraint
, which converts a quantified constraint into an entailment. This is only available when compiled with GHC 8.6 or later.
0.12 [2020.02.03]

Relax the type signature for
divideTimes
:dividesTimes :: (Divides a b, Divides a c) : Divides a (b * c) +dividesTimes :: Divides a b : Divides a (b * c)

Simplify the type signature of
dividesDef
:dividesDef :: forall a b. Divides a b : ((a * Div b a) ~ b) +dividesDef :: forall a b. Divides a b : (Mod b a ~ 0)
The original type of
diviesDef
can be (partially) recovered by defining it in terms of the newdividesDef
:dividesDef' :: forall a b. (1 <= a, Divides a b) : ((a * Div b a) ~ b) dividesDef' = Sub $ case (dividesDef @a @b, euclideanNat @a @b) of (Sub Dict, Sub Dict) > Dict
0.11.2 [2019.09.06]
 Depend on the
typeequality
compatibility library so that(:~~:)
may be used when compiling this library with GHC 8.0. This avoids having to redefine(:~~:)
directly in the internals ofconstraints
itself.
0.11.1 [2019.08.27]
 Make
Data.Constraint.Deferrable.UnsatisfiedConstraint
a newtype.
0.11 [2019.05.10]
 Introduce a
HasDict
type class for types that witness evidence of constraints, such asDict
,(:)
,Coercion
,(:~:)
,(:~~:)
, andTypeRep
.  Generalize the types of
withDict
and(\\)
to be polymorphic over anyHasDict
instance.  Add
type (⊢) = (:)
.  Fix unsafe mistakes in the statements of
dividesDef
andtimesDiv
inData.Constraint.Nat
.  Make the implementations of
Min
andMax
reduce on more inputs inData.Constraint.Nat
.  Add
minusNat
andminusZero
functions toData.Constraint.Nat
.  Support
hashable1.3.*
andsemigroups0.19.*
.
0.10.1 [2018.07.02]
 Allow building with GHC 8.6.
 Add three axioms about
(+)
and()
toData.Constraint.Nat
.
0.10
 Adapt to the
Semigroup
–Monoid
Proposal (introduced inbase4.11
): Add a
Semigroup
instance forDict
 Add the appropriate
(:=>)
instances involvingSemigroup
, and change theClass () (Monoid a)
instance toClass (Semigroup a) (Monoid a)
whenbase
is recent enough  Add the appropriate
Lifting(2)
instances involvingSemigroup
 Add a
Data.Constraint.Nat
now reexports theDiv
andMod
type families fromGHC.TypeLits
onbase4.11
or later Fix the type signature of
maxCommutes
 Export the
no
method ofBottom
 Add
NFData
instances forDict
and(:)
0.9.1
 Correct an improper use of
unsafeCoerce
in the internals ofData.Constraint.Nat
andData.Constraint.Symbol
 Correctly identify the mismatched types when you defer an unsatisfiable
equality constraint through
Data.Constraint.Deferrable
 Reexport the
(:~~:)
defined inbase
fromData.Constraint.Deferred
with GHC 8.2 or later  Add several new
(:=>)
instances forBits
,Identity
,Const
,Natural
,IO
, andWord
.  Modernize some existing
Class
and(:=>)
instances to reflect the fact thatApplicative
is now a superclass ofMonad
on recent versions ofbase
.
0.9
 Changes to
Data.Constraint
: Add
strengthen1
andstrengthen2
 Add
 Changes to
Data.Constraint.Deferrable
: Add a
Deferrable ()
instance  The
Deferrable (a ~ b)
instance now shows theTypeRep
s ofa
andb
when a type mismatch error is thrown  Add
defer_
anddeferEither_
, counterparts todefer
anddeferEither
which do not require proxy arguments  Enable
PolyKinds
. This allows theDeferrable (a ~ b)
instance to be polykinded on all supported versions of GHC except 7.10, where the kinds must be*
due to an old GHC bug  Introduce a heterogeneous equality type
(:~~:)
, and use it to define aDeferrable (a ~~ b)
instance on GHC 8.0 or later
 Add a
 Changes to
Data.Constraint.Forall
: Implement
ForallF
andForallT
in terms ofForall
 Add
ForallV
andInstV
(supporting a variable number of parameters)  Add a
forall
combinator
 Implement
 Introduce
Data.Constraint.Nat
andData.Constraint.Symbol
, which contain utilities for working withKnownNat
andKnownSymbol
constraints, respectively. These modules are only available on GHC 8.0 or later.
0.8
 GHC 8 compatibility
transformers
0.5 compatibilitybinary
0.8 compatibility Dropped support for GHC 7.6 in favor of a nicer Bottom representation.
0.7
 Found a nicer encoding of the initial object in the category of constraints using a nullary constraint.
0.6.1
 Remove the need for closed type families from the new
Forall
.
0.6
 Completely redesigned
Data.Constraint.Forall
. The old design is unsound and can be abused to defineunsafeCoerce
! The new design requires closed type families, so this module now requires GHC 7.8+
0.5.1
 Added
Data.Constraint.Deferrable
.
0.5
 Added
Data.Constraint.Lifting
.
0.4.1.3
 Acknowledge we actually need at least base 4.5
0.4.1.2
 Restore support for building on older GHCs
0.4.1.1
 Minor documentation fixes.
0.4.1
 Added
mapDict
andunmapDict
.  Added a lot of documentation.
0.4
Typeable
andData
. TheData
instance for(:)
is a conservative approximation that avoids having to turn (:) into a cartesian closed category. If it becomes a pain point for users, I know how to do that, and have done so in other libraries – see hask, but I’m hesitant to bring such heavy machinery to bear and it isn’t clear how to do it in a way that is compatible with those other libraries.
0.3.5
 Explicit role annotations
0.3.4.1
 Fixed build failures.
 Fixed an unused import warning on older GHCs.
0.3.4
 Added
bottom