Proposal: Export Data.Word.Word from Prelude

[Ben Millwood]

On Sun, Aug 17, 2014 at 07:09:21PM +0200, Daniel Díaz Casanueva wrote:
>I am +1 for this proposal.
>
>I just wanted to add one thing. Operations defined on Word are not
>non-sense or broken. In fact, IIRC, they obey the (commutative) ring
>axioms. Namely:
>
>* Word is an abelian group with respect to addition (commutative monoid
>where every element has an inverse).
>* Word is a monoid with respect to multiplication.
>* Multiplication in Word distributes the sum.
>
>Furthermore, it is commutative and unitary, with 1 being the multiplicative
>identity.

This is all quite true. However, Word, while being a ring and being 
ordered, fails to be an ordered ring, in the sense that the ordering is 
not compatible with the ring operations, so there is some 
not-insignificant inconsistency there, e.g. it is not always the case 
that `n + 1 > n`. Moreover, Word, like Int, does not have a mandated 
standard width, so while it is always a commutative ring, *which* 
commutative ring it is is ambiguous. This latter point is a very strange 
one for those who believe that types should have a clear denotation!

(All of those things are true of `Int` too, of course. But at least 
`Int` emulates `Integer` if your numbers are suitably small, which often 
they are. `Word`, on the other hand, places the threat of underflow 
*right there* next to zero, so the danger feels more difficult to avoid.)

And anyway, behaviour that is law-abiding is not necessarily helpful. We 
have to ask ourselves what actual need is fulfilled by Word. It seems to 
me that it fails as a general type for non-negative integers, because it 
does not prevent subtraction or negation. It arguably has a place in GHC 
primops, which are exotic and dangerous things that mostly should be 
hidden from the casual user. It may or may not have its place in bit 
manipulation, which I have no complaints with but basically don't 
believe happens very often, and probably is better off with types with a 
clear, specified width, like Word32.

What's left? To those enthusiastically +1-ing this proposal, how will it 
improve your lives?

>I have my doubts that we can say all these things of the type Int.
>Personally, I am not concerned about subtraction returning a bigger
>quantity: that's how modular arithmetic works and it's predictable.  But I
>understand the point made in this thread.
>
>Best regards,
>Daniel Díaz.
>
>
>On Sun, Aug 17, 2014 at 5:26 PM, Ben Millwood <haskell at benmachine.co.uk>
>wrote:
>
>> On Wed, Aug 13, 2014 at 04:25:10PM -0700, John Lato wrote:
>>
>>> On Wed, Aug 13, 2014 at 4:05 PM, Evan Laforge <qdunkan at gmail.com> wrote:
>>>
>>>  On Wed, Aug 13, 2014 at 3:06 PM, Ben Millwood <haskell at benmachine.co.uk>
>>>> wrote:
>>>> > `length :: [a] -> Word` (or things of that ilk) would be even more of a
>>>> > mistake, because type inference will spread that `Word` everywhere, and
>>>> `2 -
>>>> > 3 :: Word` is catastrophically wrong.
>>>>
>>>> This is a pretty convincing argument for me.  I have in the past used
>>>> Word for things that seemed like they should always be positive, and
>>>> pretty quickly reverted back to a signed type.  All you need is a
>>>> subtraction anywhere and the chance of underflow is very high.  And,
>>>> as Ben said, it's easy for the "always positive" type to wind up in a
>>>> domain where subtraction is valid due to type inference.  The
>>>> principled thing might be to make that a different type, but in
>>>> practice that can be a lot of hassle so it often doesn't happen (do
>>>> you use NonEmpty everywhere possible? always have separate types for
>>>> absolute and relative measures? sometimes it's not worth the clutter).
>>>>
>>>
>>>
>>> I agree it would be wrong to have `length` return a Word unconditionally,
>>> but I don't think it's a mistake in general to have that option available
>>> (i.e. genericLength).  It just means that the programmer is taking on some
>>> responsibilities manually instead of leveraging the type system, but
>>> sometimes that's the only way to get the desired performance.
>>>
>>
>> The option is already available, the proposal is to make it available in
>> the Prelude, which to me is a matter of emphasis, and what we consider
>> idiomatic or not. All this stuff about programmers taking on
>> responsibilities is perfectly fine, but I think they ought to opt-in to
>> that by importing Data.Word, rather than by having it implicitly available.
>>
>>
>>  Besides,
>>> nobody is actually proposing that `length` return a Word, so I don't find
>>> this argument relevant to the proposal.
>>>
>>
>> Quote from the original proposal:
>>
>>
>>  'Word' is usually a better choice than 'Int' when non-negative quantities
>>> (such as list lengths, bit or vector indices, or number of items in a
>>> container) need to be represented.
>>>
>>
>> If we are not proposing to use it for general non-negative things, then,
>> well, what on earth *is* it for? I mean, I imagine `length` wouldn't change
>> either way, for compatibility reasons, but either we'd use it for something
>> else, and be confronted with my argument, or we wouldn't, in which case the
>> proposal isn't really doing much good.
>>
>> I agree that there is often a need for values which must be non-negative.
>> But if we decide we need that sort of thing, we ought to worry about what
>> operations it should support and what they do in the case of underflow. It
>> seems to me like the behaviour of Word is the least Haskelly of all the
>> plausible options – not even a runtime error! – and should be confined only
>> to those who know what they are doing.
>>
>>
>>  Relatively weak +1 from me (I'm more enthusiastic about exporting the
>>> entirely of Intx/Wordx types via the prelude).
>>>
>>
>> I'm actually less against WordX and friends because at least they have a
>> precise specification and purpose. But they seem quite niche to me. What's
>> so terrible about importing them before use?
>>
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