Records in Haskell

Sun Jan 8 06:21:36 CET 2012

Thank you. I have a few questions/comments.

"The module/record ambiguity is dealt with in Frege by preferring
modules and requiring a module prefix for the record if there is
ambiguity."

I think I see why they do it this way (otherwise you can't refer to a
module if a record by the same name is in scope), but on the other
hand it would seem intuitive to me to choose the more specific thing,
and a record feels more specific than a module. Maybe you could go
that way and just not give your qualified imports the same name as a
record? (Unqualified imports are in practice going to be hierarchical,
and no one's in the habit of typing those out to disambiguate things,
so I don't think it really matters if qualified records shadow them.)

"Expressions of the form x.n: first infer the type of x. If this is
just an unbound type variable (i.e. the type is unknown yet), then
check if n is an overloaded name (i.e. a class operation). [...] Under
no circumstances, however, will the notation x.n contribute in any way
in inferring the type of x, except for the case when n is a class
operation, where an appropriate class constraint is generated."

Is this just a simple translation from x.n to n x? What's the
rationale for allowing the x.n syntax for, in addition to record
fields, class methods specifically, but no other functions?

Later on you write that the names of record fields are only accessible
from the record's namespace and via record syntax, but not from the
global scope. For Haskell I think it would make sense to reverse this
decision. On the one hand, it would keep backwards compatibility; on
the other hand, Haskell code is already written to avoid name clashes
between record fields, so it wouldn't introduce new problems. Large
gain, little pain. You could use the global-namespace function as you
do now, at the risk of ambiguity, or you could use the new record
syntax and avoid it. (If you were to also allow x.n syntax for
arbitrary functions, this could lead to ambiguity again... you could
solve it by preferring a record field belonging to the inferred type
over a function if both are available, but (at least in my current
state of ignorance) I would prefer to just not allow x.n for anything
other than record fields.)

Later on:

"- the function that updates field x of data type T is T.{x=}
- the function that sets field x in a T to 42 is T.{x=42}
- If a::T then a.{x=} and a.{x=42} are valid"

I think this looks considerably ugly. Aren't there better
alternatives? { T.x = }, { T.x = 42 }, { a.x = }, { a.x = 42 } maybe?
(Does this conflict in some unfinesseable way with explicit layout
contexts?)

"the function that changes field x of a T by applying some function to
it is T.{x <-}"

Same comment on syntax applies. I believe this is a new feature? It
would be welcome, albeit the overloading of <- is a bit worrisome
(don't have better ideas at the moment, but I think there was a
thread). I assume T.{x <- f}, a.{x <-}, and a.{x <- f} (whatever the
syntax is) would also be valid, by analogy to the above?

Re: Compatibility with existing records: based on (very) cursory
inspection I don't see an obstacle to making it (near-)fully
compatible - you would just be adding some new syntax, most
significantly x.n. Backwards compatibility is a great advantage, so
why not?

Generalizing the syntax to arbitrary TDNR: I think I'm opposed to
this. The problem is that in existing Haskell the vast majority of
expressions (with the notable (and imho unfortunate) exception of
(>>=)) flow from right to left. Going the other way with record fields
isn't a big problem because it's simple and doesn't even feel like
function application so much as member-selection (like modules), but
if you were to allow any function you would soon end up with lengthy
chains of them which would clash nastily with the surrounding code.
Having to jump back and forth and switch directions while reading is
unpleasant. OO languages have this problem and I don't envy them for
it. And in particular having "a . b" mean "first do b, then do a", but
"a.b" mean "do b to a" would be confusing. (You'd already have this
problem with global namespace record field selectors, but at least
it's localized.)

All of that said, maybe having TDNR with bad syntax is preferable to
not having TDNR at all. Can't it be extended to the existing syntax
(of function application)? Or least some better one, which is ideally
right-to-left? I don't really know the technical details...

Generalized data-namespaces: Also think I'm opposed. This would import
the problem from OO languages where functions written by the module
(class) author get to have a distinguished syntax (be inside the
namespace) over functions by anyone else (which don't).

Another thing that would be nice is lenses to solve the
nested-record-update problem - at least the room to add them later.
Most of the proposed syntax would be unaffected, but you'd need some
syntax for the lens itself... I'm not sure what it might be. Would it
be terrible to have T.x refer to a lens rather than a getter? (I don't
know how you'd refer to the getter then, so probably yeah.) Or maybe {
T.x }, building backwards from { T.x = }?

Another existing language very similar to Haskell whose record system
might be worth evaluating is Disciple: http://disciple.ouroborus.net/.
Unfortunately I couldn't find any specific page it seemed best to link
to.

On Sun, Jan 8, 2012 at 2:40 AM, Greg Weber <greg at gregweber.info> wrote:
> I have updated the wiki - the entry level page [1] compares the different
> proposals and points to a more fleshed out explanation of the Frege proposal
> [2].
>
> I think I now understand the differences between the existing proposals and
> am able to provide leadership to move this forward. Let me summarize the
> state of things:
> There is a debate over extensible records that we are putting off into the
> future. Instead we have 2 proposals to make things better right now:
> * an overloaded record fields proposal that still has implementation
> concerns
> * a name-spacing & simple type resolution proposal that is awaiting your
> critique
>
> The Frege language originally had overloaded record fields but then moved to
> the latter system. The existing experience of the Frege language is very
> fortunate for us as we now have some experience to help inform our own
> decision.
>
> Greg Weber
>
> [1] http://hackage.haskell.org/trac/ghc/wiki/Records
> [2] http://hackage.haskell.org/trac/ghc/wiki/Records/NameSpacing
>
>
> On Wed, Jan 4, 2012 at 7:54 AM, Greg Weber <greg at gregweber.info> wrote:
>>
>> The Frege author does not have a ghc mail list account but gave a more
>> detailed explanation of how he goes about TDNR for records and how often it
>> type checks without annotation in practice.
>>
>> A more general explanation is here:
>>
>> http://www.reddit.com/r/haskell/comments/nph9l/records_stalled_again_leadership_needed/c3di9sw
>>
>> He sent a specific response to Simon's mail list message, quoted below:
>>
>> Simon Peyton-Jones is absolutely correct when he notes:
>>
>> Well the most obvious issue is this. 3.2 says e.m = (T.m e) if the
>> expression e has type t and the type constructor of t is T and there exists
>> a function T.m But that innocent-looking statement begs the *entire*
>> question! How do we know if "e has type t?
>>
>> The way it is done in Frege is such that, if you have a function that uses
>> or updates (nondestructively, of course) a "record" then at least the type
>> constructor of that record has to be known. This is no different than doing
>> it explicitly with case constructs, etc., just here you learn the types from
>> the constructors you write in the patterns.
>>
>> Hence, it is not so that one can write a function that updates field f to
>> 42 for any record that contains a field f:
>>
>> foo x = x.{f=42}    -- type annotation required for foo or x
>>
>> In practice this means you'll have to write a type annotation here and
>> there.
>> Often, the field access is not the only one that happens to some variable
>> of record type, or the record is the result of another function application.
>> In such cases, the type is known.
>> I estimate that in 2/3 of all cases one does not need to write (T.e x) in
>> sparsely type annotated code, despite the fact that the frege type checker
>> has a left to right bias and does not yet attempt to find the type of x in
>> the code that "follows" the x.e construct (after let unrolling etc.)
>> I think one could do better and guarantee that, if the type of x is
>> inferrable at all, then so will be x.e (Still, it must be more than just a
>> type variable.)
>>
>> On Sun, Jan 1, 2012 at 2:39 PM, Greg Weber <greg at gregweber.info> wrote:
>>>
>>>
>>>
>>> On Sat, Dec 31, 2011 at 3:28 PM, Simon Peyton-Jones
>>> <simonpj at microsoft.com> wrote:
>>>>
>>>> Frege has a detailed explanation of the semantics of its record
>>>> implementation, and the language is *very* similar to Haskell. Lets just
>>>> start by using Frege's document as the proposal. We can start a new wiki
>>>> page as discussions are needed.
>>>>
>>>>
>>>>
>>>> If it’s a serious proposal, it needs a page to specify the design.
>>>> Currently all we have is a paragraph on
>>>> http://hackage.haskell.org/trac/ghc/wiki/Records, under “Better name
>>>> spacing”.
>>>>
>>>>
>>>>
>>>> As previously stated on this thread, the Frege user manual is available
>>>> here:
>>>>
>>>> http://code.google.com/p/frege/downloads/detail?name=Language-202.pdf
>>>>
>>>> see Sections 3.2 (primary expressions) and 4.2.1 (Algebraic Data type
>>>> Declaration - Constructors with labeled fields)
>>>>
>>>>
>>>>
>>>> To all those concerned about Records: look at the Frege implementation
>>>> and poke holes in it.
>>>>
>>>>
>>>>
>>>> Well the most obvious issue is this.  3.2 says
>>>>
>>>> e.m = (T.m e) if the expression e has type t and the type constructor
>>>>
>>>> of t is T and there exists a function T.m
>>>>
>>>> But that innocent-looking statement begs the *entire* question!  How do
>>>> we know if “e has type t?   This is the route ML takes for arithmetic
>>>> operators: + means integer plus if the argument is of type Int, float plus
>>>> if the argument is of type Float, and so on.
>>>>
>>>>
>>>>
>>>> Haskell type classes were specifically designed to address this
>>>> situation. And if you apply type classes to the record situation, I think
>>>> you end up with
>>>>
>>>> http://hackage.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields
>>>
>>>
>>> More specifically I think of this as TDNR, which instead of the focus of
>>> the wiki page of maintaining backwards compatibility and de-surgaring to
>>> polymorphic constraints. I had hoped that there were different ideas or at
>>> least more flexibility possible for the TDNR implementation.
>>>
>>>>
>>>>
>>>>
>>>> Well, so maybe we can give up on that.  Imagine Frege without the above
>>>> abbreviation.  The basic idea is that field names are rendered unique by
>>>> pre-pending the module name.  As I understand it, to record selection one
>>>> would then be forced to write (T.m e), to select the ‘m’ field.  That is
>>>> the, qualification with T is compulsory.   The trouble with this is that
>>>> it’s *already* possible; simply define suitably named fields
>>>>
>>>>   data T = MkE { t_m :: Int, t_n :: Bool }
>>>>
>>>> Here I have prefixed with a (lower case version of) the type name.  So
>>>> we don’t seem to be much further ahead.
>>>>
>>>>
>>>>
>>>> Maybe one could make it optional if there is no ambiguity, much like
>>>> Haskell’s existing qualified names.  But there is considerable ambiguity
>>>> about whether T.m means
>>>>
>>>>   m imported from module T
>>>>
>>>> or
>>>>
>>>>   the m record selector of data type T
>>>
>>>
>>> If there is ambiguity, we expect the T to be a module. So you would need
>>> to refer to Record T's module: OtherModule.T.n or T.T.n
>>> Alternatively these conflicts could be compilation errors.
>>> Either way programmers are expected to structure their programs to avoid
>>> conflicting names, no different then they do now.
>>>
>>>>
>>>>
>>>> Perhaps one could make it work out.  But before we can talk about it we
>>>> need to see a design. Which takes us back to the question of leadership.
>>>>
>>>>
>>>
>>> I am trying to provide as much leadership on this issue as I am capable
>>> of. Your critique is very useful in that effort.
>>>
>>> At this point the Frege proposal without TDNR seems to be a small step
>>> forward. We can now define records with clashing fields in the same module.
>>> However, without TDNR we don't have convenient access to those fields.
>>> I am contacting the Frege author to see if we can get any more insights
>>> on implementation details.
>>>
>>>>
>>>> Simon
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> We only want critiques about
>>>>
>>>> * achieving name-spacing right now
>>>>
>>>> * implementing it in such a way that extensible records could be
>>>> implemented in its place in the future, although we will not allow that
>>>> discussion to hold up a records implementation now, just possibly modify
>>>> things slightly.
>>>>
>>>>
>>>>
>>>> Greg Weber
>>>>
>>>>
>>>>
>>>> On Thu, Dec 29, 2011 at 2:00 PM, Simon Peyton-Jones
>>>> <simonpj at microsoft.com> wrote:
>>>>
>>>> | The lack of response, I believe, is just a lack of anyone who
>>>> | can cut through all the noise and come up with some
>>>> | practical way to move forward in one of the many possible
>>>> | directions.
>>>>
>>>> You're right.  But it is very telling that the vast majority of
>>>> responses on
>>>>
>>>>  http://www.reddit.com/r/haskell/comments/nph9l/records_stalled_again_leadership_needed/
>>>> were not about the subject (leadership) but rather on suggesting yet
>>>> more, incompletely-specified solutions to the original problem.  My modest
>>>> attempt to build a consensus by articulating the simplest solution I could
>>>> think of, manifestly failed.
>>>>
>>>> The trouble is that I just don't have the bandwidth (or, if I'm honest,
>>>> the motivation) to drive this through to a conclusion. And if no one else
>>>> does either, perhaps it isn't *that* important to anyone.  That said, it
>>>> clearly is *somewhat* important to a lot of people, so doing nothing isn't
>>>> very satisfactory either.
>>>>
>>>> Usually I feel I know how to move forward, but here I don't.
>>>>
>>>> Simon
>>>>
>>>>
>>>
>>>
>>
>
>
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