Re: [Haskell-cafe] Shadowing record field names

Any function with type `State s ()` is essentially `s -> s`. So you'd really have: modifyR :: R -> Maybe R modifyR = ... Alternatively, you can push the requirement down into the `_int` field by making the type `Even Int` for a newtype `Even` with relevant safe operations. Then, your function looks like: modifyR :: (Even Int -> Maybe (Even Int)) -> R -> Maybe R Which looks an awful lot like a `Traversal`: type Traversal s t a b = forall f. Applicative f => (a -> f b) -> s -> f t type Traversal' s a = Traversal s s a a modifyR :: Traversal (Even Int) R Matt Parsons On Sat, Dec 23, 2017 at 2:21 PM, Evan Laforge wrote:

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.

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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 wrote:

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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, wrote:

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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

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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

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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

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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

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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

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field into the typeclass instance.

On Sat, Dec 23, 2017 at 12:01 PM, Siddharth Bhat < siddu.druid@gmail.com> wrote:

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At the risk of derailing the thread, what exactly does it mean to be

an

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"orphan instance"? And where does "#bar" come from, I've never seen

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syntax before :) I followed the exposition up to that point, if it helps.

Thanks, Siddharth

On Sun 24 Dec, 2017, 01:23 Tikhon Jelvis, wrote:

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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

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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

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`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

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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

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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

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much perfect: fields are first-class citizens (as lenses) and are not in the same scope as identifiers. We've been using this extensively

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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 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

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>> 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

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>> 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

On Sat, Dec 23, 2017 at 12:31 PM, Tikhon Jelvis wrote: program*. that there is the that like directly things if pretty throughout our the code. than mainly with

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>> more abstract things. >> >> What do people do to address this problem? Any recommendations or >> best practices? >> >> Josh >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. >> > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post.

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