Some clarity please! (was Re: [Haskell-cafe] Re: (flawed?) benchmark : sort)

Conor McBride conor at
Thu Mar 13 21:42:28 EDT 2008


On 13 Mar 2008, at 23:42, kahl at wrote:

> Conor McBride <conor at> 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]
Prelude Data.List> sortBy (\_ _ -> GT) [4,5,3,2,1]

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


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