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:
  • The Bifunctor type class, introduced in base-4.8.0.0
  • The Bifoldable and Bitraversable type classes, introduced in base-4.10.0.0
• 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:
  • The Identity data type, introduced in base-4.8.0.0
  • The MonadIO type class; and the Compose, Product, and Sum data types, introduced in base-4.9.0.0
• 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!