[GHC] #15509: `showEFloat` inconsistency introduced in base-4.12

[GHC]

#15509: `showEFloat` inconsistency introduced in base-4.12
-------------------------------------+-------------------------------------
        Reporter:  hvr               |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1-beta1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by hvr:

Old description:

> While base-4.11 had a `showEFloat` that behaved like
>
> {{{#!hs
> showEFloat (Just n) | n <= 0 == showEFloat (Just 1)
> }}}
>
> For non-positive values, base-4.12 introduced an unfortunate
> inconsistency:
>
> {{{#!hs
> showEFloat (Just n) | n < 0 == showEFloat (Just 1)
> -- NB: *not* equivalent to     showEFloat (Just 0)
> }}}
>
> e.g. now we have
>
> {{{#!hs
> showEFloat (Just 2)    1.0 "" == "1.00e0"
> showEFloat (Just 1)    1.0 "" == "1.0e0"
> showEFloat (Just 0)    1.0 "" == "1e0"
> showEFloat (Just (-1)) 1.0 "" == "1.0e0"
> showEFloat (Just (-2)) 1.0 "" == "1.0e0"
> }}}
>
> iow, negative `n`s in the precision argument are now treated differently
> from `(Just 0)` which is a source of subtle bugs for API consumers which
> assume `showEFloat` to have "continuous total" semantics for the
> precision argument.

New description:

 While base-4.11 had a `showEFloat` that behaved like

 {{{#!hs
 showEFloat (Just n) | n <= 0 == showEFloat (Just 1)
 }}}

 For non-positive values, base-4.12 introduced an unfortunate
 inconsistency:

 {{{#!hs
 showEFloat (Just n) | n < 0 == showEFloat (Just 1)
 -- NB: *not* equivalent to     showEFloat (Just 0)
 }}}

 e.g. now we have

 {{{#!hs
 showEFloat (Just 2)    1.0 "" == "1.00e0"
 showEFloat (Just 1)    1.0 "" == "1.0e0"
 showEFloat (Just 0)    1.0 "" == "1e0"
 showEFloat (Just (-1)) 1.0 "" == "1.0e0"
 showEFloat (Just (-2)) 1.0 "" == "1.0e0"
 }}}

 iow, negative `n`s in the precision argument are now treated differently
 from `(Just 0)` which is a source of subtle bugs for API consumers which
 assume `showEFloat` to have "continuous total" (or rather, saturated
 subtraction) semantics for the precision argument.

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15509#comment:2>
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