Changelog for singletons2.6
Changelog for singletons project
2.6

Require GHC 8.8.

Sing
has switched from a data family to a type family. This has a number of consequences:
Names like
SBool
,SMaybe
, etc. are no longer type synonyms for particular instantiations ofSing
but are instead the names of the singleton data types themselves. In other words, previous versions ofsingletons
would provide this:data instance Sing :: Bool > Type where SFalse :: Sing False STrue :: Sing True type SBool = (Sing :: Bool > Type)
Whereas with
Sing
asatypefamily,singletons
now provides this:data SBool :: Bool > Type where SFalse :: SBool False STrue :: SBool True type instance Sing @Bool = SBool

The
Sing
instance forTYPE rep
inData.Singletons.TypeRepTYPE
is now directly defined astype instance Sing @(TYPE rep) = TypeRep
, without the use of an intermediate newtype as before. 
Due to limitations in the ways that quantified constraints and type families can interact (see this GHC issue), the internals of
ShowSing
has to be tweaked in order to continue to work withSing
asatypefamily. One notable consequence of this is thatShow
instances for singleton types can no longer be derived—they must be written by hand in order to work around this GHC bug. This is unlikely to affect you unless you define 'Show' instances for singleton types by hand. For more information, refer to the Haddocks forShowSing'
inData.Singletons.ShowSing
. 
GHC does not permit type class instances to mention type families, which means that it is no longer possible to define instances that mention the
Sing
type constructor. For this reason, aWrappedSing
data type (which is a newtype aroundSing
) was introduced so that one can hang instances off of it.This had one noticeable effect in
singletons
itself: there are no longerTestEquality Sing
orTestCoercion Sing
instances. Instead,singletons
now generates a separateTestEquality
/TestCoercion
instance for every data type that singles a derivedEq
instance. In addition, theData.Singletons.Decide
module now provides topleveldecideEquality
/decideCoercion
functions which provide the behavior oftestEquality
/testCoercion
, but monomorphized toSing
. Finally,TestEquality
/TestCoercion
instances are provided forWrappedSing
.


GHC's behavior surrounding kind inference for local definitions has changed in 8.8, and certain code that
singletons
generates for local definitions may no longer typecheck as a result. While we have taken measures to mitigate the issue onsingletons
' end, there still exists code that must be patched on the users' end in order to continue compiling. For instance, here is an example of code that stopped compiling with the switch to GHC 8.8:replicateM_ :: (Applicative m) => Nat > m a > m () replicateM_ cnt0 f = loop cnt0 where loop cnt  cnt <= 0 = pure ()  otherwise = f *> loop (cnt  1)
This produces errors to the effect of:
• Could not deduce (SNum k1) arising from a use of ‘sFromInteger’ from the context: SApplicative m ... • Could not deduce (SOrd k1) arising from a use of ‘%<=’ from the context: SApplicative m ...
The issue is that GHC 8.8 now kindgeneralizes
sLoop
(whereas it did not previously), explaining why the error message mentions a mysterious kind variablek1
that only appeared after kind generalization. The solution is to giveloop
an explicit type signature like so:replicateM_ :: (Applicative m) => Nat > m a > m () +replicateM_ :: forall m a. (Applicative m) => Nat > m a > m () replicateM_ cnt0 f = loop cnt0 where + loop :: Nat > m () loop cnt  cnt <= 0 = pure ()  otherwise = f *> loop (cnt  1)
This general approach should be sufficient to fix any type inference regressions that were introduced between GHC 8.6 and 8.8. If this isn't the case, please file an issue.

Due to GHC Trac #16133 being fixed,
singletons
generated code now requires explicitly enabling theTypeApplications
extension. (The generated code was always usingTypeApplications
under the hood, but it's only now that GHC is detecting it.) 
Data.Singletons
now defines a family ofSingI
instances forTyCon1
throughTyCon8
:instance (forall a. SingI a => SingI (f a), ...) => SingI (TyCon1 f) instance (forall a b. (SingI a, SingI b) => SingI (f a b), ...) => SingI (TyCon2 f) ...
As a result,
singletons
no longer generates instances forSingI
instances for applications ofTyCon{N}
to particular type constructors, as they have been superseded by the instances above. 
Changes to
Data.Singletons.Sigma
:
SSigma
, the singleton type forSigma
, is now defined. 
New functions
fstSigma
,sndSigma
,FstSigma
,SndSigma
,currySigma
, anduncurrySigma
have been added. AShow
instance forSigma
has also been added. 
projSigma1
has been redefined to use continuationpassing style to more closely resemble its cousinprojSigma2
. The new type signature ofprojSigma1
is:projSigma1 :: (forall (fst :: s). Sing fst > r) > Sigma s t > r
The old type signature of
projSigma1
can be found in thefstSigma
function. 
Σ
has been redefined such that it is now a partial application ofSigma
, like so:type Σ = Sigma
One benefit of this change is that one no longer needs defunctionalization symbols in order to partially apply
Σ
. As a result,ΣSym0
,ΣSym1
, andΣSym2
have been removed.


In line with corresponding changes in
base4.13
, theFail
/sFail
methods of{P,S}Monad
have been removed in favor of new{P,S}MonadFail
classes introduced in theData.Singletons.Prelude.Monad.Fail
module. These classes are also reexported fromData.Singletons.Prelude
. 
Fix a bug where expressions with explicit signatures involving function types would fail to single.

The infix names
(.)
and(!)
are no longer mapped to(:.)
and(:!)
, as GHC 8.8 learned to parse them at the type level. 
The
Enum
instance forSomeSing
now uses more efficient implementations ofenumFromTo
andenumFromThenTo
that no longer require aSingKind
constraint.
2.5.1
ShowSing
is now a type class (with a single instance) instead of a type synonym. This was changed because definingShowSing
as a type synonym prevents it from working well with recursive types due to an unfortunate GHC bug. For more information, see issue #371. Add an
IsString
instance forSomeSing
.
2.5

The
Data.Promotion.Prelude.*
namespace has been removed. Use the corresponding modules in theData.Singletons.Prelude.*
namespace instead. 
Fix a regression in which certain infix type families, such as
(++)
,($)
,(+)
, and others, did not have the correct fixities. 
The default implementation of the
(==)
type inPEq
was changed from(Data.Type.Equality.==)
to a custom type family,DefaultEq
. The reason for this change is that(Data.Type.Equality.==)
is unable to conclude thata == a
reduces toTrue
for anya
. (As a result, the previous version ofsingletons
regressed in terms of type inference for thePEq
instances forNat
andSymbol
, which used that default.) On the other hand,DefaultEq a a
does reduce toTrue
for alla
. 
Add
Enum Nat
,Show Nat
, andShow Symbol
instances toData.Singletons.TypeLits
. 
Template Haskellgenerated code may require
DataKinds
andPolyKinds
in scenarios which did not previously require it:singletons
now explicitly quantifies all kind variables used in explicitforall
s.singletons
now generatesa ~> b
instead ofTyFun a b > Type
whenever possible.

Since
thdesugar
now desugars all data types to GADT syntax, Template Haskellgenerated code may requireGADTs
in situations that didn't require it before. 
Overhaul the way derived
Show
instances for singleton types works. Before, there was an awkwardShowSing
class (which was essentially a cargoculted version ofShow
specialized forSing
) that one had to create instances for separately. Now that GHC hasQuantifiedConstraints
, we can scrap this whole class and turnShowSing
into a simple type synonym:type ShowSing k = forall z. Show (Sing (z :: k))
Now, instead of generating a handwritten
ShowSing
andShow
instance for each singleton type, we only generate a single (derived!)Show
instance. As a result of this change, you will likely need to enableQuantifiedConstraints
andStandaloneDeriving
if you single any derivedShow
instances in your code. 
The kind of the type parameter to
SingI
is no longer specified. This only affects you if you were using thesing
method withTypeApplications
. For instance, if you were usingsing @Bool @True
before, then you will now need to now usesing @Bool
instead. 
singletons
now generatesSingI
instances for defunctionalization symbols through Template Haskell. As a result, you may need to enableFlexibleInstances
in more places. 
genDefunSymbols
is now more robust with respect to types that use dependent quantification, such as:type family MyProxy k (a :: k) :: Type where MyProxy k (a :: k) = Proxy a
See the documentation for
genDefunSymbols
for limitations to this. 
Rename
Data.Singletons.TypeRepStar
toData.Singletons.TypeRepTYPE
, and generalize theSing :: Type > Type
instance toSing :: TYPE rep > Type
, allowing it to work over more open kinds. Also renameSomeTypeRepStar
toSomeTypeRepTYPE
, and change its definition accordingly. 
Promoting or singling a type synonym or type family declaration now produces defunctionalization symbols for it. (Previously, promoting or singling a type synonym did nothing whatsoever, and promoting or singling a type family produced an error.)

singletons
now produces fixity declarations for defunctionalization symbols when appropriate. 
Add
(%<=?)
, a singled version of(<=?)
fromGHC.TypeNats
, as well as defunctionalization symbols for(<=?)
, toData.Singletons.TypeLits
. 
Add
Data.Singletons.Prelude.{Semigroup,Monoid}
, which define promoted and singled versions of theSemigroup
andMonoid
type classes, as well as various newtype modifiers.Symbol
is now has promotedSemigroup
andMonoid
instances as well. As a consequence,Data.Singletons.TypeLits
no longer exports(<>)
or(%<>)
, as they are superseded by the corresponding methods fromPSemigroup
andSSemigroup
. 
Add promoted and singled versions of the
Functor
,Foldable
,Traversable
,Applicative
,Alternative
,Monad
,MonadPlus
, andMonadZip
classes. Among other things, this grants the ability to promote or singledo
notation and list comprehensions.Data.Singletons.Prelude.List
now reexports more generalFoldable
/Traversable
functions wherever possible, just asData.List
does.

Add
Data.Singletons.Prelude.{Const,Identity}
, which define promoted and singled version of theConst
andIdentity
data types, respectively. 
Promote and single the
Down
newtype inData.Singletons.Prelude.Ord
. 
To match the
base
library, the promoted/singled versions ofcomparing
andthenCmp
are no longer exported fromData.Singletons.Prelude
. (They continue to live inData.Singletons.Prelude.Ord
.) 
Permit singling of expression and pattern signatures.

Permit promotion and singling of
InstanceSigs
. 
sError
andsUndefined
now haveHasCallStack
constraints, like their counterpartserror
andundefined
. The promoted and singled counterparts toerrorWithoutStackTrace
have also been added in case you do not want this behavior. 
Add
Data.Singletons.TypeError
, which provides a dropin replacement forGHC.TypeLits.TypeError
which can be used at both the value and typelevel.
2.4.1
 Restore the
TyCon1
,TyCon2
, etc. types. It turns out that the newTyCon
doesn't work with kindpolymorphic tycons.
2.4

Require GHC 8.4.

Demote Nat
is nowNatural
(fromNumeric.Natural
) instead ofInteger
. In accordance with this change,Data.Singletons.TypeLits
now exposesGHC.TypeNats.natVal
(which returns aNatural
) instead ofGHC.TypeLits.natVal
(which returns anInteger
). 
The naming conventions for infix identifiers (e.g.,
(&*)
) have been overhauled.
Infix functions (that are not constructors) are no longer prepended with a colon when promoted to type families. For instance, the promoted version of
(&*)
is now called(&*)
as well, instead of(:&*)
as before.There is one exception to this rule: the
(.)
function, which is promoted as(:.)
. The reason is that one cannot write(.)
at the type level. 
Singletons for infix functions are now always prepended with
%
instead of%:
. 
Singletons for infix classes are now always prepended with
%
instead of:%
. 
Singletons for infix datatypes are now always prepended with a
%
.(Before, there was an unspoken requirement that singling an infix datatype required that name to begin with a colon, and the singleton type would begin with
:%
. But now that infix datatype names can be things like(+)
, this requirement became obsolete.)
The upshot is that most infix names can now be promoted using the same name, and singled by simply prepending the name with
%
. 

The suffix for defunctionalized names of symbolic functions (e.g.,
(+)
) has changed. Before, the promoted type name would be suffixed with some number of dollar signs (e.g.,(+$)
and(+$$)
) to indicate defunctionalization symbols. Now, the promoted type name is first suffixed with@#@
and then followed by dollar signs (e.g.,(+@#@$)
and(+@#@$$)
). Adopting this conventional eliminates naming conflicts that could arise for functions that consisted of solely$
symbols. 
The treatment of
undefined
is less magical. Before, all uses ofundefined
would be promoted toGHC.Exts.Any
and singled toundefined
. Now, there is a properUndefined
type family andsUndefined
singleton function. 
As a consequence of not promoting
undefined
toAny
, there is no need to have a specialany_
function to distinguish the function on lists. The corresponding promoted type, singleton function, and defunctionalization symbols are now namedAny
,sAny
, andAnySym{0,1,2}
. 
Rework the treatment of empty data types:
 Generated
SingKind
instances for empty data types now useEmptyCase
instead of simplyerror
ing.  Derived
PEq
instances for empty data types now returnTrue
instead ofFalse
. DerivedSEq
instances now returnTrue
instead oferror
ing.  Derived
SDecide
instances for empty data types now returnProved bottom
, wherebottom
is a divergent computation, instead oferror
ing.
 Generated

Add
Data.Singletons.Prelude.IsString
andData.Promotion.Prelude.IsString
modules.IsString.fromString
is now used when promoting or singling string literals when theXOverloadedStrings
extension is enabled (similarly to howNum.fromInteger
is currently used when promoting or singling numeric literals). 
Add
Data.Singletons.Prelude.Void
. 
Add promoted and singled versions of
div
,mod
,divMod
,quot
,rem
, andquotRem
toData.Singletons.TypeLits
that utilize the efficientDiv
andMod
type families fromGHC.TypeNats
. Also addsLog2
and defunctionalization symbols forLog2
fromGHC.TypeNats
. 
Add
(<>)
and(%<>)
, the promoted and singled versions ofAppendSymbol
fromGHC.TypeLits
. 
Add
(%^)
, the singleton version ofGHC.TypeLits.^
. 
Add
unlines
andunwords
toData.Singletons.Prelude.List
. 
Add promoted and singled versions of
Show
, includingderiving
support. 
Add a
ShowSing
class, which facilitates the ability to writeShow
instances forSing
instances. 
Permit derived
Ord
instances for empty datatypes. 
Permit standalone
deriving
declarations. 
Permit
DeriveAnyClass
(through theanyclass
keyword ofDerivingStrategies
) 
Add a valuelevel
(@@)
, which is a synonym forapplySing
. 
Add
Eq
,Ord
,Num
,Enum
, andBounded
instances forSomeSing
, which leverage theSEq
,SOrd
,SNum
,SEnum
, andSBounded
instances, respectively, for the underlyingSing
. 
Rework the
Sing (a :: *)
instance inData.Singletons.TypeRepStar
such that it now uses typeindexedTypeable
. The newSing
instance is now:newtype instance Sing :: Type > Type where STypeRep :: TypeRep a > Sing a
Accordingly, the
SingKind
instance has also been changed:instance SingKind Type where type Demote Type = SomeTypeRepStar ... data SomeTypeRepStar where SomeTypeRepStar :: forall (a :: *). !(TypeRep a) > SomeTypeRepStar
Aside from cleaning up some implementation details, this change assures that
toSing
can only be called onTypeRep
s whose kind is of kind*
. The previous implementation did not enforce this, which could lead to segfaults if used carelessly. 
Instead of
error
ing, thetoSing
implementation in theSingKind (k1 ~> k2)
instance now works as one would expect (provided the user adheres to some commonsenseSingKind
laws, which are now documented). 
Add a
demote
function, which is a convenient shorthand forfromSing sing
. 
Add a
Data.Singletons.Sigma
module with aSigma
(dependent pair) data type. 
Export defunctionalization symbols for
Demote
,SameKind,
KindOf,
(~>),
Apply, and
(@@)from
Data.Singletons`. 
Add an explicitly bidirectional pattern synonym
Sing
. Pattern matching onSing
brings aSingI ty
constraint into scope from a singletonSing ty
. 
Add an explicitly bidirectional pattern synonym
FromSing
. Pattern matching on any demoted (base) type gives us the corresponding singleton. 
Add explicitly bidirectional pattern synonyms
SLambda{2..8}
. Pattern matching on any defunctionalized singleton yields a termlevel Haskell function on singletons. 
Remove the family of
TyCon1
,TyCon2
, ..., in favor of justTyCon
. GHC 8.4's type system is powerful enough to allow this nice simplification.
2.3

Documentation clarifiation in
Data.Singletons.TypeLits
, thanks to @ivanm. 
Demote
was no longer a convenient way of callingDemoteRep
and has been removed.DemoteRep
has been renamedDemote
. 
DemoteRep
is now injective. 
Demoting a
Symbol
now givesText
. This is motivated by makingDemoteRep
injective. (IfSymbol
demoted toString
, then there would be a conflict between demoting[Char]
andSymbol
.) 
Generating singletons also now generates fixity declarations for the singletonized definitions, thanks to @intindex.

Though more an implementation detail: singletons no longer uses kindlevel proxies anywhere, thanks again to @intindex.

Support for promoting higherkinded type variables, thanks for @intindex.

Data.Singletons.TypeLits
now exports defunctionalization symbols forKnownNat
andKnownSymbol
. 
Better type inference support around constraints, as tracked in Issue #176.

Type synonym definitions are now ignored, as they should be.

Show
instances forSNat
andSSymbol
, thanks to @cumber. 
The
singFun
andunSingFun
functions no longer use proxies, preferringTypeApplications
.
2.2

With
TypeInType
, we no longer kindKProxy
. @intindex has very helpfully removed the use ofKProxy
fromsingletons
. 
Drop support for GHC 7.x.

Remove
bugInGHC
. That function was intended to work around GHC's difficulty in detecting exhaustiveness of GADT pattern matches. GHC 8 comes with a much better exhaustiveness checker, and so this function is no longer necessary.
2.1

Require
thdesugar
>= 1.6 
Work with GHC 8. GHC 8 gives the opportunity to simplify some pieces of singletons, but these opportunities are not yet fully realized. For example, injective type families means that we no longer need
Sing
to be a data family; it could be a type family. This might drastically simplify the way functions are singletonized. But not yet! 
singletons
now outputs a few more type/kind annotations to help GHC do type inference. There may be a few more programs accepted than before. (This is the fix for #136.)
2.0.1
 Lots more functions in
Data.Singletons.Prelude.List
:filter
,find
,elemIndex
,elemIndices
,findIndex
,findIndices
,intersect
,intersectBy
,takeWhile
,dropWhile
,dropWhileEnd
,span
,break
,take
,drop
,splitAt
,group
,maximum
,minimum
,insert
,sort
,groupBy
,lookup
,partition
,sum
,product
,length
,replicate
,transpose
,(!!)
,nub
,nubBy
,unionBy
,union
,genericLength
2.0.0.2
 Fix fixity of
*
.
2.0.0.1
 Make haddock work.
2.0

Instance promotion now works properly  it was quite buggy in 1.0.

Classes and instances can now be singletonized.

Limited support for functional dependencies.

We now have promoted and singletonized versions of
Enum
, as well asBounded
. 
Deriving
Enum
is also now supported. 
Ditto for
Num
, which includes an instance forNat
, naturally. 
Promoting a literal number now uses overloaded literals at the type level, using a typelevel
FromInteger
in the typelevelNum
class. 
Better support for dealing with constraints. Some previouslyunsingletonizable functions that have constrained parameters now work.

No more orphan
Quasi
instances! 
Support for functions of arity 8 (instead of the old limit, 7).

Full support for fixity declarations.

A raft of bugfixes.

Drop support for GHC 7.8. You must have GHC 7.10.2.
1.1.2.1
Fix bug #116, thus allowing locallydeclared symbols to be used in GHC 7.10.
1.1.2
 No more GHC 7.8.2 support  you must have GHC 7.8.3.
1.1.1
Update testsuite to work with thdesugar1.5.2. No functional changes.
1.1
This is a maintenance release to support building (but not testing, due to
GHC bug #10058) with 7.10. This release also targets thdesugar1.5. Some
types changed (using thdesugar's new DsMonad
instead of Quasi
), but
clients generally won't need to make any changes, unless they, too, generalize
over Quasi
.
1.0
This is a complete rewrite of the package.

A much wider array of surface syntax is now accepted for promotion and singletonization, including
let
,case
, partiallyapplied functions, and anonymous functions,where
, sections, among others. 
Classes and instances can be promoted (but not singletonized).

Derivation of promoted instances for
Ord
andBounded
.
This release can be seen as a "technology preview". More features are coming soon.
This version drops GHC 7.6 support.
0.10.0
Template Haskell names are now more hygienic. In other words, singletons
won't try to gobble up something happened to be named Sing
in your project.
(Note that the Template Haskell names are not completely hygienic; names
generated during singleton generation can still cause conflicts.)
If a function to be promoted or singletonized is missing a type signature, that is now an error, not a warning.
Added a new external module Data.Singletons.TypeLits, which contain the singletons for GHC.TypeLits. Some convenience functions are also provided.
The extension EmptyCase
is no longer needed. This caused pain when trying
to support both GHC 7.6.3 and 7.8.
0.9.3
Fix export list of Data.Singletons.TH, again again.
Add SEq
instances for Nat
and Symbol
.
0.9.2
Fix export list of Data.Singletons.TH, again.
0.9.1
Fix export list of Data.Singletons.TH.
0.9.0
Make compatible with GHC HEAD, but HEAD reports core lint errors sometimes.
Change module structure significantly. If you want to derive your own
singletons, you should import Data.Singletons.TH
. The module
Data.Singletons
now exports functions only for the use of singletons.
New modules Data.Singletons.Bool
, ...Maybe
, ...Either
, and ...List
are just like their equivalents from Data.
, except for List
, which is
quite lacking in features.
For singleton equality, use Data.Singletons.Eq
.
For propositional singleton equality, use Data.Singletons.Decide
.
New module Data.Singletons.Prelude
is meant to mirror the Haskell Prelude,
but with singleton definitions.
Streamline representation of singletons, resulting in exponential speedup at execution. (This has not been rigorously measured, but the data structures are now exponentially smaller.)
Add internal support for TypeLits, because the TypeLits module no longer exports singleton definitions.
Add support for existential singletons, through the toSing
method of
SingKind
.
Remove the SingE
class, bundling its functionality into SingKind
.
Thus, the SingRep
synonym has also been removed.
Name change: KindIs
becomes KProxy
.
Add support for singletonizing calls to error
.
Add support for singletonizing empty data definitions.
0.8.6
Make compatible with GHC HEAD, but HEAD reports core lint errors sometimes.
0.8.5
Bug fix to make singletons compatible with GHC 7.6.1.
Added git info to cabal file.
0.8.4
Update to work with latest version of GHC (7.7.20130114).
Now use branched type family instances to allow for promotion of functions with overlapping patterns.
Permit promotion of functions with constraints by omitting constraints.
0.8.3
Update to work with latest version of GHC (7.7.20121031).
Removed use of Any to simulate kind classes; now using KindOf and OfKind from GHC.TypeLits.
Made compatible with GHC.TypeLits.
0.8.2
Added this changelog
Update to work with latest version of GHC (7.6.1). (There was a change to Template Haskell).
Moved library into Data.Singletons.
0.8.1
Update to work with latest version of GHC. (There was a change to Template Haskell).
Updated dependencies in cabal to include the newer version of TH.
0.8
Initial public release