# 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 of Sing but are instead the names of the singleton data types themselves. In other words, previous versions of singletons would provide this:

data instance Sing :: Bool -> Type where
SFalse :: Sing False
STrue  :: Sing True
type SBool = (Sing :: Bool -> Type)


Whereas with Sing-as-a-type-family, singletons now provides this:

data SBool :: Bool -> Type where
SFalse :: SBool False
STrue  :: SBool True
type instance Sing @Bool = SBool

• The Sing instance for TYPE rep in Data.Singletons.TypeRepTYPE is now directly defined as type 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 with Sing-as-a-type-family. One notable consequence of this is that Show 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 for ShowSing' in Data.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, a WrappedSing data type (which is a newtype around Sing) was introduced so that one can hang instances off of it.

This had one noticeable effect in singletons itself: there are no longer TestEquality Sing or TestCoercion Sing instances. Instead, singletons now generates a separate TestEquality/TestCoercion instance for every data type that singles a derived Eq instance. In addition, the Data.Singletons.Decide module now provides top-level decideEquality/decideCoercion functions which provide the behavior of testEquality/testCoercion, but monomorphized to Sing. Finally, TestEquality/TestCoercion instances are provided for WrappedSing.

• 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 on singletons' 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 kind-generalizes sLoop (whereas it did not previously), explaining why the error message mentions a mysterious kind variable k1 that only appeared after kind generalization. The solution is to give loop 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 the TypeApplications extension. (The generated code was always using TypeApplications under the hood, but it's only now that GHC is detecting it.)

• Data.Singletons now defines a family of SingI instances for TyCon1 through TyCon8:

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 for SingI instances for applications of TyCon{N} to particular type constructors, as they have been superseded by the instances above.

• Changes to Data.Singletons.Sigma:

• SSigma, the singleton type for Sigma, is now defined.

• New functions fstSigma, sndSigma, FstSigma, SndSigma, currySigma, and uncurrySigma have been added. A Show instance for Sigma has also been added.

• projSigma1 has been redefined to use continuation-passing style to more closely resemble its cousin projSigma2. The new type signature of projSigma1 is:

projSigma1 :: (forall (fst :: s). Sing fst -> r) -> Sigma s t -> r


The old type signature of projSigma1 can be found in the fstSigma function.

• Σ has been redefined such that it is now a partial application of Sigma, 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, and ΣSym2 have been removed.

• In line with corresponding changes in base-4.13, the Fail/sFail methods of {P,S}Monad have been removed in favor of new {P,S}MonadFail classes introduced in the Data.Singletons.Prelude.Monad.Fail module. These classes are also re-exported from Data.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 for SomeSing now uses more efficient implementations of enumFromTo and enumFromThenTo that no longer require a SingKind constraint.

## 2.5.1

• ShowSing is now a type class (with a single instance) instead of a type synonym. This was changed because defining ShowSing 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 for SomeSing.

## 2.5

• The Data.Promotion.Prelude.* namespace has been removed. Use the corresponding modules in the Data.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 in PEq 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 that a == a reduces to True for any a. (As a result, the previous version of singletons regressed in terms of type inference for the PEq instances for Nat and Symbol, which used that default.) On the other hand, DefaultEq a a does reduce to True for all a. • Add Enum Nat, Show Nat, and Show Symbol instances to Data.Singletons.TypeLits. • Template Haskell-generated code may require DataKinds and PolyKinds in scenarios which did not previously require it: • singletons now explicitly quantifies all kind variables used in explicit foralls. • singletons now generates a ~> b instead of TyFun a b -> Type whenever possible. • Since th-desugar now desugars all data types to GADT syntax, Template Haskell-generated code may require GADTs in situations that didn't require it before. • Overhaul the way derived Show instances for singleton types works. Before, there was an awkward ShowSing class (which was essentially a cargo-culted version of Show specialized for Sing) that one had to create instances for separately. Now that GHC has QuantifiedConstraints, we can scrap this whole class and turn ShowSing into a simple type synonym: type ShowSing k = forall z. Show (Sing (z :: k))  Now, instead of generating a hand-written ShowSing and Show instance for each singleton type, we only generate a single (derived!) Show instance. As a result of this change, you will likely need to enable QuantifiedConstraints and StandaloneDeriving if you single any derived Show instances in your code. • The kind of the type parameter to SingI is no longer specified. This only affects you if you were using the sing method with TypeApplications. For instance, if you were using sing @Bool @True before, then you will now need to now use sing @Bool instead. • singletons now generates SingI instances for defunctionalization symbols through Template Haskell. As a result, you may need to enable FlexibleInstances 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 to Data.Singletons.TypeRepTYPE, and generalize the Sing :: Type -> Type instance to Sing :: TYPE rep -> Type, allowing it to work over more open kinds. Also rename SomeTypeRepStar to SomeTypeRepTYPE, 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 (<=?) from GHC.TypeNats, as well as defunctionalization symbols for (<=?), to Data.Singletons.TypeLits. • Add Data.Singletons.Prelude.{Semigroup,Monoid}, which define promoted and singled versions of the Semigroup and Monoid type classes, as well as various newtype modifiers. Symbol is now has promoted Semigroup and Monoid instances as well. As a consequence, Data.Singletons.TypeLits no longer exports (<>) or (%<>), as they are superseded by the corresponding methods from PSemigroup and SSemigroup. • Add promoted and singled versions of the Functor, Foldable, Traversable, Applicative, Alternative, Monad, MonadPlus, and MonadZip classes. Among other things, this grants the ability to promote or single do-notation and list comprehensions. • Data.Singletons.Prelude.List now reexports more general Foldable/Traversable functions wherever possible, just as Data.List does. • Add Data.Singletons.Prelude.{Const,Identity}, which define promoted and singled version of the Const and Identity data types, respectively. • Promote and single the Down newtype in Data.Singletons.Prelude.Ord. • To match the base library, the promoted/singled versions of comparing and thenCmp are no longer exported from Data.Singletons.Prelude. (They continue to live in Data.Singletons.Prelude.Ord.) • Permit singling of expression and pattern signatures. • Permit promotion and singling of InstanceSigs. • sError and sUndefined now have HasCallStack constraints, like their counterparts error and undefined. The promoted and singled counterparts to errorWithoutStackTrace have also been added in case you do not want this behavior. • Add Data.Singletons.TypeError, which provides a drop-in replacement for GHC.TypeLits.TypeError which can be used at both the value- and type-level. ## 2.4.1 • Restore the TyCon1, TyCon2, etc. types. It turns out that the new TyCon doesn't work with kind-polymorphic tycons. ## 2.4 • Require GHC 8.4. • Demote Nat is now Natural (from Numeric.Natural) instead of Integer. In accordance with this change, Data.Singletons.TypeLits now exposes GHC.TypeNats.natVal (which returns a Natural) instead of GHC.TypeLits.natVal (which returns an Integer). • 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 of undefined would be promoted to GHC.Exts.Any and singled to undefined. Now, there is a proper Undefined type family and sUndefined singleton function.

• As a consequence of not promoting undefined to Any, there is no need to have a special any_ function to distinguish the function on lists. The corresponding promoted type, singleton function, and defunctionalization symbols are now named Any, sAny, and AnySym{0,1,2}.

• Rework the treatment of empty data types:

• Generated SingKind instances for empty data types now use EmptyCase instead of simply erroring.
• Derived PEq instances for empty data types now return True instead of False. Derived SEq instances now return True instead of erroring.
• Derived SDecide instances for empty data types now return Proved bottom, where bottom is a divergent computation, instead of erroring.
• Add Data.Singletons.Prelude.IsString and Data.Promotion.Prelude.IsString modules. IsString.fromString is now used when promoting or singling string literals when the -XOverloadedStrings extension is enabled (similarly to how Num.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, and quotRem to Data.Singletons.TypeLits that utilize the efficient Div and Mod type families from GHC.TypeNats. Also add sLog2 and defunctionalization symbols for Log2 from GHC.TypeNats.

• Add (<>) and (%<>), the promoted and singled versions of AppendSymbol from GHC.TypeLits.

• Add (%^), the singleton version of GHC.TypeLits.^.

• Add unlines and unwords to Data.Singletons.Prelude.List.

• Add promoted and singled versions of Show, including deriving support.

• Add a ShowSing class, which facilitates the ability to write Show instances for Sing instances.

• Permit derived Ord instances for empty datatypes.

• Permit standalone deriving declarations.

• Permit DeriveAnyClass (through the anyclass keyword of DerivingStrategies)

• Add a value-level (@@), which is a synonym for applySing.

• Add Eq, Ord, Num, Enum, and Bounded instances for SomeSing, which leverage the SEq, SOrd, SNum, SEnum, and SBounded instances, respectively, for the underlying Sing.

• Rework the Sing (a :: *) instance in Data.Singletons.TypeRepStar such that it now uses type-indexed Typeable. The new Sing 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 on TypeReps whose kind is of kind *. The previous implementation did not enforce this, which could lead to segfaults if used carelessly.

• Instead of erroring, the toSing implementation in the SingKind (k1 ~> k2) instance now works as one would expect (provided the user adheres to some common-sense SingKind laws, which are now documented).

• Add a demote function, which is a convenient shorthand for fromSing sing.

• Add a Data.Singletons.Sigma module with a Sigma (dependent pair) data type.

• Export defunctionalization symbols for Demote, SameKind, KindOf, (~>), Apply, and (@@)fromData.Singletons.

• Add an explicitly bidirectional pattern synonym Sing. Pattern matching on Sing brings a SingI ty constraint into scope from a singleton Sing 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 term-level Haskell function on singletons.

• Remove the family of TyCon1, TyCon2, ..., in favor of just TyCon. GHC 8.4's type system is powerful enough to allow this nice simplification.

## 2.3

• Documentation clarifiation in Data.Singletons.TypeLits, thanks to @ivan-m.

• Demote was no longer a convenient way of calling DemoteRep and has been removed. DemoteRep has been renamed Demote.

• DemoteRep is now injective.

• Demoting a Symbol now gives Text. This is motivated by making DemoteRep injective. (If Symbol demoted to String, then there would be a conflict between demoting [Char] and Symbol.)

• Generating singletons also now generates fixity declarations for the singletonized definitions, thanks to @int-index.

• Though more an implementation detail: singletons no longer uses kind-level proxies anywhere, thanks again to @int-index.

• Support for promoting higher-kinded type variables, thanks for @int-index.

• Data.Singletons.TypeLits now exports defunctionalization symbols for KnownNat and KnownSymbol.

• Better type inference support around constraints, as tracked in Issue #176.

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

• Show instances for SNat and SSymbol, thanks to @cumber.

• The singFun and unSingFun functions no longer use proxies, preferring TypeApplications.

## 2.2

• With TypeInType, we no longer kind KProxy. @int-index has very helpfully removed the use of KProxy from singletons.

• 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 th-desugar >= 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

• 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 as Bounded.

• Deriving Enum is also now supported.

• Ditto for Num, which includes an instance for Nat, naturally.

• Promoting a literal number now uses overloaded literals at the type level, using a type-level FromInteger in the type-level Num class.

• Better support for dealing with constraints. Some previously-unsingletonizable 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 locally-declared 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 th-desugar-1.5.2. 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 th-desugar-1.5. Some types changed (using th-desugar'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, partially-applied functions, and anonymous functions, where, sections, among others.

• Classes and instances can be promoted (but not singletonized).

• Derivation of promoted instances for Ord and Bounded.

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

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

Removed use of Any to simulate kind classes; now using KindOf and OfKind from GHC.TypeLits.