[Haskell-cafe] Shadowing record field names

Siddharth Bhat siddu.druid at gmail.com
Sat Dec 23 20:21:55 UTC 2017

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?


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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