A compatibility layer for base
Scope
The scope of base-compat
is to provide functions available in later versions
of base to a wider (older) range of compilers.
In addition, successful library proposals that have been accepted to be part of
upcoming versions of base
are also included. This package is not intended to
replace base
, but to complement it.
Note that base-compat
does not add any orphan instances. There is a separate
package base-orphans
for
that.
In addition, base-compat
only backports functions. In particular, we
purposefully do not backport data types or type classes introduced in newer
versions of base
. For more info, see the
Data types and type classes
section.
base-compat
is intentionally designed to have zero dependencies. As a
consequence, there are some modules that can only be backported up to certain
versions of base
. If an even wider support window is desired in these
scenarios, there also exists a base-compat-batteries
package which augments
base-compat
with certain compatibility package dependencies. For more info,
see the Dependencies section.
Basic usage
In your cabal file, you should have something like this:
build-depends: base >= 4.3
, base-compat >= 0.9.0
Then, lets say you want to use the isRight
function introduced with
base-4.7.0.0
. Replace:
import Data.Either
with
import Data.Either.Compat
Note (1): There is no need to import both unqualified. The .Compat
modules
re-exports the original module.
Note (2): If a given module .Compat
version is not defined, that either
means that:
- The module has not changed in recent base versions, thus no
.Compat
is
needed.
- The module has changed, but the changes depend on newer versions of GHC, and
thus are not portable.
- The module has changed, but those changes have not yet been merged in
base-compat
: patches are welcomed!
Using Prelude.Compat
If you want to use Prelude.Compat
(which provides all the
AMP/Traversable/Foldable changes from base-4.8.0.0
), it’s best to hide
Prelude
, e.g.:
import Prelude ()
import Prelude.Compat
main :: IO ()
main = mapM_ print (Just 23)
Alternatively, you can use the NoImplicitPrelude
language extension:
{-# LANGUAGE NoImplicitPrelude #-}
import Prelude.Compat
main :: IO ()
main = mapM_ print (Just 23)
Note that we use
mapM_ :: (Foldable t, Monad m) => (a -> m b) -> t a -> m ()
from Data.Foldable
here, which is only exposed from Prelude
since
base-4.8.0.0
.
Using this approach allows you to write code that works seamlessly with all
versions of GHC that are supported by base-compat
.
What is covered
So far the following is covered.
For compatibility with the latest released version of base
Prelude.Compat
incorporates the AMP/Foldable/Traversable changes and
exposes the same interface as Prelude
from base-4.9.0.0
System.IO.Error.catch
is not re-exported from Prelude.Compat
for older
versions of base
Text.Read.Compat.readMaybe
Text.Read.Compat.readEither
Data.Monoid.Compat.<>
- Added
bitDefault
, testBitDefault
, and popCountDefault
to Data.Bits.Compat
- Added
toIntegralSized
to Data.Bits.Compat
(if using base-4.7
)
- Added
bool
function to Data.Bool.Compat
- Added
isLeft
, isRight
, fromLeft
, and fromRight
to Data.Either.Compat
- Added
forkFinally
to Control.Concurrent.Compat
- Added
withMVarMasked
function to Control.Concurrent.MVar.Compat
- Added
(<$!>)
function to Control.Monad.Compat
- Weakened
RealFloat
constraints on realPart
, imagPart
, conjugate
, mkPolar
,
and cis
in Data.Complex.Compat
- Added more efficient
maximumBy
/minimumBy
to Data.Foldable.Compat
- Added
($>)
and void
functions to Data.Functor.Compat
(&)
function to Data.Function.Compat
($>)
and void
functions to Data.Functor.Compat
modifyIORef'
, atomicModifyIORef'
and atomicWriteIORef
to Data.IORef.Compat
dropWhileEnd
, isSubsequenceOf
, sortOn
, and uncons
functions to Data.List.Compat
- Correct versions of
nub
, nubBy
, union
, and unionBy
to Data.List.Compat
asProxyTypeOf
with a generalized type signature to Data.Proxy.Compat
modifySTRef'
to Data.STRef.Compat
String
, lines
, words
, unlines
, and unwords
to Data.String.Compat
gcoerceWith
to Data.Type.Coercion.Compat
makeVersion
function to Data.Version.Compat
traceId
, traceShowId
, traceM
, and traceShowM
functions to Debug.Trace.Compat
byteSwap16
, byteSwap32
, and byteSwap64
to Data.Word.Compat
plusForeignPtr
to Foreign.ForeignPtr.Compat
calloc
and callocBytes
functions to Foreign.Marshal.Alloc.Compat
callocArray
and callocArray0
functions to Foreign.Marshal.Array.Compat
fillBytes
to Foreign.Marshal.Utils.Compat
- Added
Data.List.Compat.scanl'
showFFloatAlt
and showGFloatAlt
to Numeric.Compat
lookupEnv
, setEnv
and unsetEnv
to System.Environment.Compat
unsafeFixIO
and unsafeDupablePerformIO
to System.IO.Unsafe.IO
RuntimeRep
-polymorphic ($!)
to Prelude.Compat
RuntimeRep
-polymorphic throw
to Control.Exception.Compat
What is not covered
Data types and type classes
base-compat
purposefully does not export any data types or type classes that
were introduced in more recent versions of base
. The main reasoning for this
policy is that it is not some data types and type classes have had their APIs
change in different versions of base
, which makes having a consistent
compatibility API for them practically impossible.
As an example, consider the FiniteBits
type class. It was introduced in
base-4.7.0.0
with the following API:
class Bits b => FiniteBits b where
finiteBitSize :: b -> Int
However, in base-4.8.0.0
,
FiniteBits
gained additional functions:
class Bits b => FiniteBits b where
finiteBitSize :: b -> Int
countLeadingZeros :: b -> Int -- ^ @since 4.8.0.0
countTrailingZeros :: b -> Int -- ^ @since 4.8.0.0
This raises the question: how can FiniteBits
be backported consistently
across all versions of base
? One approach is to backport the API exposed in
base-4.8.0.0
on versions prior to 4.7.0.0
. The problem with this is that
countLeadingZeros
and countTrailingZeros
are not exposed in base-4.7.0.0
,
so instances of FiniteBits
would have to be declared like this:
instance FiniteBits Foo where
finiteBitSize = ...
#if MIN_VERSION_base(4,8,0) || !(MIN_VERSION_base(4,7,0))
countLeadingZeros = ...
countTrailingZeros = ...
#endif
Another approach is to backport the API from base-4.7.0.0
and to declare
additional methods outside of the class:
#if MIN_VERSION_base(4,7,0) && !(MIN_VERSION_base(4,8,0))
countLeadingZeros :: FiniteBits b => b -> Int
countLeadingZeros = {- default implementation #-}
#endif
The situation is only slightly better for classes which exist across all versions of base
,
but have grown their API. For example, it’s tempting to define
#if !(MIN_VERSION_base(4,8,0))
displayException :: Exception e => e -> String
displayException = show
#endif
As with the previous approach, you won’t be able to define new members of the type
class without CPP guards. In other words, the non-CPP approach would limit
uses to the lowest common denominator.
As neither approach is a very satisfactory solution, and to embrace
consistency, we do not pursue either approach. For similar reasons, we do not
backport data types.
Dependencies
base-compat
is designed to have zero dependencies (besides libraries that
ship with GHC itself). A consequence of this choice is that there are certain
modules that have a “limited” support window. An important example of this is
Prelude.Compat
, which backports the Semigroup
class to versions of base
older than 4.11 (when it was added to the Prelude
). Because Semigroup
was
not added to base
until base-4.9
, base-compat
cannot backport it to
any earlier version of base
than this.
If you would instead desire to be able to use a version of Prelude.Compat
that does backport Semigroup
to even older versions of base
, even if it
means pulling in other dependencies, then you are in luck. There also exists
a base-compat-batteries
package, which exposes a strict superset of the API
in base-compat
. base-compat-batteries
has all the same modules as
base-compat
, but exposes more functionality on more versions of base
by
reexporting things from compatibility libraries whenever necessary. (For
instance, base-compat-batteries
exports the Semigroup
class from the
semigroups
library when built against versions of base
older than 4.9.)
Because base-compat
and base-compat-batteries
have the same module names,
they are quite easy to switch out for one another in library projects, at the
expense of having clashing names if one tries to import them in GHCi. To
work around this issue, base-compat
and base-compat-batteries
also provide
copies of each module with the suffix .Repl
(for base-compat
) and
.Repl.Batteries
(for base-compat-batteries
) to give them globally unique
namespaces in the event one wants to import them into GHCi.
Here is a list of compatibility libraries that base-compat-batteries
depends
on, paired with the things that each library backports:
bifunctors
for:
contravariant
for the Contravariant
type class, introduced in base-4.12.0.0
.
fail
for the MonadFail
type class, introduced in base-4.9.0.0
nats
for the Natural
data type, introduced in base-4.8.0.0
semigroups
for the Semigroup
typeclass and the
NonEmpty
,
Min
,
Max
,
First
,
Last
,
WrappedMonoid
,
Option
,
and
Arg
data types, introduced in base-4.9.0.0
tagged
for the Proxy
data type, introduced in base-4.7.0.0
transformers
for:
void
for the Void
data type, introduced in base-4.8.0.0
Supported versions of GHC/base
ghc-8.6.1
/ base-4.12.0.0
ghc-8.4.4
/ base-4.11.1.0
ghc-8.4.3
/ base-4.11.1.0
ghc-8.4.2
/ base-4.11.1.0
ghc-8.4.1
/ base-4.11.0.0
ghc-8.2.2
/ base-4.10.1.0
ghc-8.2.1
/ base-4.10.0.0
ghc-8.0.2
/ base-4.9.1.0
ghc-8.0.1
/ base-4.9.0.0
ghc-7.10.3
/ base-4.8.2.0
ghc-7.10.2
/ base-4.8.1.0
ghc-7.10.1
/ base-4.8.0.0
ghc-7.8.4
/ base-4.7.0.2
ghc-7.8.3
/ base-4.7.0.1
ghc-7.8.2
/ base-4.7.0.0
ghc-7.8.1
/ base-4.7.0.0
ghc-7.6.3
/ base-4.6.0.1
ghc-7.6.2
/ base-4.6.0.1
ghc-7.6.1
/ base-4.6.0.0
ghc-7.4.2
/ base-4.5.1.0
ghc-7.4.1
/ base-4.5.0.0
ghc-7.2.2
/ base-4.4.1.0
ghc-7.2.1
/ base-4.4.0.0
ghc-7.0.4
/ base-4.3.1.0
ghc-7.0.3
/ base-4.3.1.0
ghc-7.0.2
/ base-4.3.1.0
ghc-7.0.1
/ base-4.3.0.0
We also make an attempt to keep base-compat
building with GHC HEAD, but due
to its volatility, it may not work at any given point in time. If it doesn’t,
please report it!
Patches are welcome; add tests for new code!