[Haskell-cafe] Shadowing record field names

Tikhon Jelvis tikhon at jelv.is
Sat Dec 23 20:31:06 UTC 2017

Yes, that is a good summary. Typeclass instances are inherently not
modular. There are some tradeoffs to this design—if you want to see an
alternative, take a look at how Scala deals with implicits.

On Sat, Dec 23, 2017 at 12:21 PM, Siddharth Bhat <siddu.druid at gmail.com>

> Ah, thank you! I was unaware of the  "OverloadedLabels" extension.
> So, orphan instances are an anti-pattern because their scope is hard to
> control? As in, when we import a module, we cannot "choose" to import
> typeclass definitions on types, right?
> Thanks,
> Siddharth
> On Sun 24 Dec, 2017, 01:46 Tikhon Jelvis, <tikhon at jelv.is> wrote:
>> Please don't worry about derailing—if you had the question, I'm sure a
>> lot of other people reading did as well.
>> An "orphan instance" is a typeclass instance defined in a module that
>> doesn't also define the class or the type. In my example, it's an instance
>> of the IsLabel class (from GHC.OverloadedLabels) for the function type (a
>> -> b). This is a problem because Haskell typeclass instances are not
>> module—a type can only have one instance of a given class *in the entire
>> program*. This means that if two libraries defined the instance I gave,
>> *you would not be able to use them together in the same project*. This is
>> why you should absolutely not define this instance in a library.
>> An alternative design here would be to define a new type for lenses that
>> does not overlap with (a -> b). Something like ReifiedLens, defined
>> specifically to have this instance:
>> newtype FieldLens s t a b = FieldLens (Lens s t a b)
>> instance (...) => IsLabel (FieldLens s t a b) where
>>   ...
>> Unfortunately, this would make FieldLens incompatible with normal lenses,
>> leading to a bit of boilerplate each time you had to call a field.
>> The other alternative is to use RebindableSyntax which lets you
>> substitute your own IsLabel class in place of the one defined in
>> GHC.OverloadedLabels. This is probably the neatest solution, but
>> RebindableSyntax feels like a really heavyweight extension to use. That
>> said, my guess is that we'll use it in our internal code at work if there
>> is ever a conflict with the current IsLabel instance we're using. The
>> current experience with records is too nice to pass up without a fight :).
>> The #bar syntax uses the OverloadedLabels extension. This adds the
>> IsLabel class and desugars #bar into fromLabel @"bar"
>> The @ is type application, so the "bar" in fromLabel @"bar" is a
>> type-level symbol, not a normal string. This is how we get the name of the
>> field into the typeclass instance.
>> On Sat, Dec 23, 2017 at 12:01 PM, Siddharth Bhat <siddu.druid at gmail.com>
>> wrote:
>>> At the risk of derailing the thread, what exactly does it mean to be an
>>> "orphan instance"? And where does "#bar" come from, I've never seen that
>>> syntax before :) I followed the exposition up to that point, if it helps.
>>> Thanks,
>>> Siddharth
>>> On Sun 24 Dec, 2017, 01:23 Tikhon Jelvis, <tikhon at jelv.is> wrote:
>>>> This is a real pain point with records in Haskell.
>>>> The fundamental problem is that unlike most languages with records or
>>>> object, field names are treated as normal identifiers in Haskell. Other
>>>> languages make fields special—you can only use them with the .
>>>> operator or in other select contexts. The advantage is that you can do
>>>> things like `a.author == author`; the disadvantage is that fields become a
>>>> second-class citizen.
>>>> At work, we have a solution that's really nice to use built on top of
>>>> DuplicateRecordFields and OverloadedLabels. Our approach follows the ideas
>>>> in the OverloadedRecordFields proposal but with a lens flavor—very
>>>> similar to the overloaded-records[1] package. (We don't use that package
>>>> directly because I wrote our own version before I knew about it and I like
>>>> the ergonomics of our internal version a bit more.)
>>>> We have a couple of typeclasses for reading and writing fields:
>>>> class HasField (field :: Symbol) s a | s -> a where
>>>>   getField :: s -> a
>>>> class UpdatesField (field :: Symbol) s t b | name t -> b, name s b -> t
>>>> where
>>>>   updateField :: s -> b -> t
>>>> A record field can be both read and updated:
>>>> type Field field s t a b = (HasField field s a, UpdatesField field name
>>>> s t b)
>>>> field :: forall (name :: Symbol) s t a b. Field name s t a b => Lens s
>>>> t a b
>>>> field = lens (getField @name) (updateField @name)
>>>> Then we have some Template Haskell for generating instances of these
>>>> classes. Here's a contrived example:
>>>> data Foo a = Foo { bar :: [a] }
>>>> record ''Foo
>>>> which generates:
>>>> instance HasField "bar" (Foo a) a where
>>>>   getField = bar
>>>> instance UpdatesField "bar" (Foo a) (Foo b) b where
>>>>   updateField foo bar' = foo { bar = bar' }
>>>> Given these, we can already write code looking up fields as lenses:
>>>> > Foo [1,2,3] ^. field @"bar"
>>>> [1,2,3]
>>>> Now fields aren't normal identifiers any more, the names can be shared
>>>> over different records (with DuplicateRecordFields) and you can write
>>>> functions polymorphic over any record with a given field.
>>>> The names and details here are a bit different, but I believe this is
>>>> otherwise exactly what overloaded-records gives you. You could also replace
>>>> the TH to generate instances with generics in the style of the generic-lens
>>>> library.
>>>> However, the field @"bar" is painfully verbose. We solve this using
>>>> OverloadedLabels and a somewhat shady orphan instance for IsLabel:
>>>> instance (Functor f, Field name s t a b, a' ~ (a -> f b), b' ~ (s -> f
>>>> t)) => IsLabel name (a' -> b') where
>>>>   fromLabel = field @name
>>>> The details are a bit fiddly, but this is what we need to make type
>>>> inference work correctly. This lets us replace field @"name" with #name:
>>>> > Foo [1,2,3] ^. #bar
>>>> [1,2,3]
>>>> > Foo [1,2,3] & #bar . each %~ show
>>>> Foo { bar = ["1","2","3"] }
>>>> The downside is that this is an orphan instance for IsLabel for *all
>>>> functions*. You would not want to use this in a library but it's fine in an
>>>> executable as long as you don't mind potentially needing to reword things
>>>> if a similar IsLabel instance is added to base. (A risk I'm willing to take
>>>> for better syntax :))
>>>> Apart from that (somewhat serious) downside, the final result is pretty
>>>> much perfect: fields are first-class citizens (as lenses) and are not in
>>>> the same scope as identifiers. We've been using this extensively throughout
>>>> our whole project and it's been perfect—perhaps surprisingly, we haven't
>>>> run into any issues with type inference or type error messages (beyond what
>>>> you normally get with lens).
>>>> With this addition, Haskell records went from being a real blemish on
>>>> the language to being the best I've ever used. The orphan instance is a
>>>> definite red flag and you should absolutely *not* have that instance in a
>>>> library, but if you're working on a standalone executable or some extensive
>>>> internal code, I think it's absolutely worth it.
>>>> [1]: https://hackage.haskell.org/package/overloaded-records
>>>> [2]: https://hackage.haskell.org/package/generic-lens
>>>> On Sat, Dec 23, 2017 at 6:41 AM, Li-yao Xia <lysxia at gmail.com> wrote:
>>>>> I don't think "authorL" hurts readability. It just seems the logical
>>>>> choice if "author" is already taken.
>>>>> Have you seen generic-lens? The lens for the "author" field is  (field
>>>>> @"author") so there is some added noise compared to "authorL", but it can
>>>>> be used as a TH-free alternative to makeClassy.
>>>>> type Field name a = forall s. HasField name s s a a => Lens s s a a
>>>>> authorL :: Field "author" Author
>>>>> authorL = field @"author"
>>>>> Cheers,
>>>>> Li-yao
>>>>> On 12/23/2017 08:36 AM, ☂Josh Chia (謝任中) wrote:
>>>>>> Quite often, I need to use record types like this:
>>>>>> data Whole1 = Whole1 { part :: Part, ... }
>>>>>> data Whole2 = Whole2 { part :: Part, ... }
>>>>>> Where Whole1 & Whole2 are types of things that have a Part and some
>>>>>> other things. E.g. a Book has an Author, a Title, etc and so does an
>>>>>> Article.
>>>>>> The problem is that I'm not actually allowed to use the same name
>>>>>> (author/part) in two different record types. Some people use lens to solve
>>>>>> this. You can have a lens called 'author' for dealing with the Author in
>>>>>> both Book and Article (e.g. using makeClassy).
>>>>>> That's fine, but there's yet another problem. Let's say I have a
>>>>>> function that takes an Author and a [Library] and returns all the Libraries
>>>>>> that have Books or Articles matching the Author. So:
>>>>>> findAuthorLibraries :: Author -> [Library] -> [Library]
>>>>>> findAuthorLibraries author libraries = ...
>>>>>> But I already have a lens called 'author' and ghc will complain about
>>>>>> shadowing. So, to avoid shadowing, should I use 'theAuthor' instead of
>>>>>> 'author' for the function argument? Or, should I name the lens
>>>>>> 'authorLens', 'authorL' or 'lAuthor' instead of 'author'? Prefixing with
>>>>>> 'the' is quite unreadable because whether or not an argument has that
>>>>>> prefix depends on whether there's a lens with a conflicting name so it adds
>>>>>> noise to the code. Adding a 'Lens' prefix to the 'author' lens also seems
>>>>>> quite an overbearing eyesore because for consistency I would have to use
>>>>>> the prefix for all my field-accessing lenses.
>>>>>> Maybe I should use Lens.Control.TH.makeClassy and then define:
>>>>>> findAuthorLibraries :: HasAuthor a => a -> [Library] -> [Library]
>>>>>> findAuthorLibraries hasAuthor libraries = ...
>>>>>> But that may be making my function more complicated and general than
>>>>>> I want, affecting readability, simplicity, compilation time and maybe even
>>>>>> performance.
>>>>>> In summary, I find that there are ways around the problem but they
>>>>>> really affect readability.
>>>>>> I could also disable the warning about shadowing but that seems
>>>>>> pretty dangerous. It may be OK to disable the warning for the specific
>>>>>> cases where a function argument shadows something from the topmost scope,
>>>>>> but GHC does not allow such selective disabling of that warning.
>>>>>> In a code base that deals mainly with concrete business logic, this
>>>>>> problem probably crops up more than in a code base that deals mainly with
>>>>>> more abstract things.
>>>>>> What do people do to address this problem? Any recommendations or
>>>>>> best practices?
>>>>>> Josh
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>>> --
>>> Sending this from my phone, please excuse any typos!
>> --
> Sending this from my phone, please excuse any typos!
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