#7135: Data.Bits can still have default implementations for testBit, bit, and
popCount by using -XDefaultSignatures
------------------------------+---------------------------------------------
Reporter: joeyadams | Owner:
Type: feature request | Status: new
Priority: normal | Component: Runtime System
Version: | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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In commit f99cee0 of the base package, the Num constraint was removed from
the Bits class. Because the default implementations of testBit, bit, and
popCount required Num, the same commit made them external functions
instead.
We can still have default implementations for these if we use the
-XDefaultSignatures GHC extension:
{{{
#ifdef __GLASGOW_HASKELL__
default bit :: Num a => Int -> a
bit = bitDefault
default testBit :: Num a => a -> Int -> Bool
testBit = testBitDefault
default popCount :: Num a => a -> Int
popCount = popCountDefault
#endif
}}}
Whether we want to do this or not, I'm not sure.
Pro:
* This is more convenient.
* This may prevent people from encountering runtime errors in existing
code that does not have bit, testBit, and popCount implementations, where
the author forgot to compile with -Wall .
Con:
* Users get away with writing less portable code, so those who do wish to
use non-GHC will have more support work to do.
The attached patch adds defaults for testBit, bit, and popCount, for GHC
only. It does not update the documentation, though, so "Minimal complete
definition" as-is would be misleading.
P.S.: This ticket is for GHC 7.6.1-rc1. That version tag is not
available, though.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7135>
GHC <http://www.haskell.org/ghc/>
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