-
The singletons library has been split into three libraries:
- The new
singletons library is now a minimal library that only provides
Data.Singletons, Data.Singletons.Decide, Data.Singletons.Sigma, and
Data.Singletons.ShowSing (if compiled with GHC 8.6 or later).
singletons now supports building GHCs back to GHC 8.0, as well as GHCJS.
- The
singletons-th library defines Template Haskell functionality for
promoting and singling term-level definitions, but but nothing else. This
library continues to require the latest stable release of GHC.
- The
singletons-base library defines promoted and singled versions of
definitions from the base library, including the Prelude. This library
continues to require the latest stable release of GHC.
Consult the changelogs for singletons and singletons-base for changes
specific to those libraries. For more information on this split, see the
relevant GitHub discussion.
-
Require building with GHC 9.0.
-
Data.Singletons.CustomStar and Data.Singletons.SuppressUnusedWarnings
have been renamed to Data.Singletons.TH.CustomStar and
Data.Singletons.SuppressUnusedWarnings, respectively, to give every module
in singletons-th a consistent module prefix.
-
Due to the singletons package split, the singletons-th modules
Data.Singletons.TH and Data.Singletons.TH.CustomStar (formerly known as
Data.Singletons.CustomStar) no longer re-export any definitions from the
singletons-base module Prelude.Singletons (formerly known as
Data.Singletons.Prelude). The singletons-base library now provides
versions of these modules—Data.Singletons.Base.CustomStar and
Data.Singletons.Base.TH, respectively—that do re-export definitions
from Prelude.Singletons.
-
“Fully saturated” defunctionalization symbols (e.g., IdSym1) are now
defined as type families instead of type synonyms. This has two notable
benefits:
- Fully saturated defunctionalization symbols can now be given standalone
kind signatures, which ensures that the order of kind variables is the
same as the user originally declared them.
- This fixes a minor regression in
singletons-2.7 in which the quality
of :kind! output in GHCi would become worse when using promoted type
families generated by Template Haskell.
Under certain circumstances, this can be a breaking change:
-
Since more TH-generated promoted functions now have type families on
their right-hand sides, some programs will now require
UndecidableInstances where they didn’t before.
-
Certain definitions that made use of overlapping patterns, such as
natMinus below, will no longer typecheck:
$(singletons [d|
data Nat = Z | S Nat
natMinus :: Nat -> Nat -> Nat
natMinus Z _ = Z
natMinus (S a) (S b) = natMinus a b
natMinus a Z = a
|])
This can be worked around by avoiding the use of overlapping patterns.
In the case of natMinus, this amounts to changing the third equation
to match on its first argument:
$(singletons [d|
natMinus :: Nat -> Nat -> Nat
natMinus Z _ = Z
natMinus (S a) (S b) = natMinus a b
natMinus a@(S _) Z = a
|])
-
The specification for how singletons deals with record selectors has been
simplified. Previously, singletons would try to avoid promoting so-called
“naughty” selectors (those whose types mention existential type variables
that do not appear in the constructor’s return type) to top-level functions.
Determing if a selector is naughty is quite challenging in practice, as
determining if a type variable is existential or not in the context of
Template Haskell is difficult in the general case. As a result, singletons
now adopts the dumb-but-predictable approach of always promoting record
selectors to top-level functions, naughty or not.
This means that attempting to promote code with a naughty record selector,
like in the example below, will no longer work:
$(promote [d|
data Some :: (Type -> Type) -> Type where
MkSome :: { getSome :: f a } -> Some f
-- getSome is naughty due to mentioning the type variable `a`
|])
Please open an issue if you find this restriction burdensome in practice.
-
The singEqInstanceOnly and singEqInstancesOnly functions, which generate
SEq (but not PEq) instances, have been removed. There is not much point
in keeping these functions around now that PEq now longer has a special
default implementation. Use singEqInstance{s} instead.
-
The Template Haskell machinery will no longer promote TypeRep to Type,
as this special case never worked properly in the first place.
-
The Template Haskell machinery will now preserve strict fields in data types
when generating their singled counterparts.
-
Introduce a new promotedDataTypeOrConName option to
Data.Singletons.TH.Options. Overriding this option can be useful in
situations where one wishes to promote types such as Nat, Symbol, or
data types built on top of them. See the
“Arrows, Nat, Symbol, and literals” section of the README for more
information.
-
Define a Quote instance for OptionsM. A notable benefit of this instance
is that it avoids the need to explicitly lift TH quotes into OptionsM.
Before, you would have to do this:
import Control.Monad.Trans.Class (lift)
withOptions defaultOptions
$ singletons
$ lift [d| data T = MkT |]
But now, it suffices to simply do this:
withOptions defaultOptions
$ singletons [d| data T = MkT |]
