Hi
On 13 Mar 2008, at 23:42, [EMAIL PROTECTED] wrote:
Conor McBride <[EMAIL PROTECTED]> responded to my comment:
(mapMonotonic should of course be removed, too,
or specified to fail (preferably in some MonadZero)
if the precondition is violated,
which should still be implementable in linear time.)
What's wrong with mapMonotonic that isn't wrong
with, say, sortBy?, or Eq instances for parametrized
types?
Prelude> :m + Data.Set
Prelude Data.Set> toAscList $ mapMonotonic (10 -) (fromList [1 .. 5])
[9,8,7,6,5]
Prelude Data.Set> 5 `member` mapMonotonic (10 -) (fromList [1 .. 5])
False
Something's certainly wrong there!
But nothing out of the ordinary: garbage in,
garbage out. Happens all the time, even in
Haskell. Why pick on mapMonotonic?
Prelude Data.List> sortBy (\_ _ -> GT) [1,3,2,5,4]
[4,5,2,3,1]
Prelude Data.List> sortBy (\_ _ -> GT) [4,5,3,2,1]
[1,2,3,5,4]
I guess there's a question of what we might
call "toxic waste"---junk values other than
undefined. I think undefined is bad enough
already. So the type system can't express
the spec. I don't think we should be casual
about that: we should be precise in
documentation about the obligations which
fall on the programmer. Some dirt is
pragmatically necessary: we shouldn't pretend
that it ain't so; we shouldn't pretend that dirt
is clean.
Before we can talk about what == should return,
can we settle what we mean by = ?
``='' is not in the Haskell interface! ;-)
No, but "is" is in the human interface!
Therefore, I talked only about (==).
Ah, but you talked about things. Which
things? Is one of the things you talked
about the same as (==)? the same as
(flip (==))?
The best way to include ``='' seems to be the semantic equality of
P-logic
[Harrison-Kieburtz-2005], which is quite a heavy calibre,
and at least in that paper, classes are not yet included.
I expect it's hard work. It's hard work in
much better behaved systems. My point is that
it's worth it, in order to facilitate more
meaningful discussions.
All the best
Conor
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