#5525: Mismatch between documentation and behaviour of encodeFloat
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Reporter: daniel.is.fischer | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries/base
Version: 7.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Documentation bug
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In the process of elaborating the docs to treat #3898, I wrote
{{{
-- | 'encodeFloat' performs the inverse of 'decodeFloat' in the
-- sense that for finite @x@ with the exception of @-0.0@,
-- @'uncurry' 'encodeFloat' ('decodeFloat' x) = x@.
-- @'encodeFloat' m n@ is the closest representable floating-point
-- number to @m*b^^n@ (or @±Infinity@ if overflow occurs).
}}}
Unfortunately, the last sentence is not quite true. If the end result is a
subnormal number, `encodeFloat` performs two roundings, which may lead to
a wrong result (can be 1 ulp off). Changing `encodeFloat` so that it
always finds the closest representable number would cost a lot in
performance, it would bring it close to `fromRational`. So the thing to do
is change the docs, I just don't know how yet.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5525>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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