[Haskell-cafe] Shadowing record field names

Evan Laforge qdunkan at gmail.com
Sat Dec 23 21:21:08 UTC 2017

Here's another semi-derail: how can you do get/set with side-effects?
This would necessarily be for a record in StateM or something.  For
instance, say to preserve an invariant on a field:

    import qualified Control.Monad.State as State

    data R { _int :: Int }

    modifyInt :: (R -> R) -> State.State R ()
    modifyInt modify = do
        int <- modify <$> State.gets _int
        if even int then State.modify \r -> r { _int = int }
            else Except.throwError "odd"

I couldn't figure out how to get this sort of thing to cooperate with
lenses.  I can make a lens with effects easily enough, but you have to
invoke it via 'State.modify $ ...', at which point you're exposing
State.modify which defeats the idea of trying to enforce invariants.

On Sat, Dec 23, 2017 at 12:31 PM, Tikhon Jelvis <tikhon at jelv.is> wrote:
> 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>
> wrote:
>> 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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