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Types as Interfaces

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Re: Types as Interfaces

#11

Post by derdi »

posix_monad wrote: post #3

MLs require a lot of ceremony modelling simple record types. What we want to express here is an object with a map of properties (name to type): string Type map For the OOP minded: Map And also compose those: type Foo = { "_foo", int } type Bar = { "_bar", string } type FooBar = mergeMaps Foo Bar But at compile-time, of course. Have any languages achieved this? I know TypeScript can do some of these things, but it's c…

OCaml's first-class modules allow you to do this: https://ocaml.org/play#code=bW9kdWxlIHR5cGUgRk9PID0gc2lnCiAg...

Re: Types as Interfaces

#12

Post by pyrale »

posix_monad wrote: post #3

MLs require a lot of ceremony modelling simple record types. What we want to express here is an object with a map of properties (name to type): string Type map For the OOP minded: Map And also compose those: type Foo = { "_foo", int } type Bar = { "_bar", string } type FooBar = mergeMaps Foo Bar But at compile-time, of course. Have any languages achieved this? I know TypeScript can do some of these things, but it's c…

Haskell does it just fine with the HasField typeclass, but like everything Haskell, it's not always easy to find the information.

    type HasFooBar s = (HasField "_foo" s String, HasField "_bar" s String) => s

Re: Types as Interfaces

#13

Post by js8 »

brabel wrote: post #2

Is this something professional programmers are having trouble with?? This is the kind of problem you face in your first year working, no? I am honetly curious what others think. Do you have trouble deciding when to use an interface (assuming your language has that), or a type wrapper (I don't think that's the brightest idea), or a function to extract the field to sort by (most languages do that)??

> Is this something professional programmers are having trouble with??

I would say, yes. So does everybody else. If you don't believe it, I don't think you appreciate the depth of these problems.

I am reminded of Eric Meijer saying that the interface (in the Java sense) part of the contract between types is the least interesting part. What is important are their algebraic properties, which can be modeled using functional types and other bunch of advanced type-theoretical ideas (like linear types for instance).

Modelling stuff with types is not easy, it's an art.

Re: Types as Interfaces

#14

Post by brabel »

Sakos wrote: post #7
brabel wrote: post #2

Is this something professional programmers are having trouble with?? This is the kind of problem you face in your first year working, no? I am honetly curious what others think. Do you have trouble deciding when to use an interface (assuming your language has that), or a type wrapper (I don't think that's the brightest idea), or a function to extract the field to sort by (most languages do that)??

> Is this something professional programmers are having trouble with?? Nobody said this. There is value in thinking about things like this sometimes, because it has long-term consequences for the projects we work on. Even if you're a "professional" programmer (whatever that means), it's valuable to go back to beliefs and knowledge you've established long ago to evaluate whether to change them in the face of new exper…

Why scare quotes though? Professional programmer just means someone who writes software for a living. Software is your profession... there's no doubt what that means , is there??

If you do that, you'll have run into this sort of decision very early in your career, and hopefully will understand the best way to handle it, which IMHO just depends on the language (because certain features lead to different optimum solutions) and the culture around it. But sure, I am happy to discuss fundamental topics like this, that's why I am engaging and asking what others think.

Re: Types as Interfaces

#15

Post by pyrale »

noelwelsh wrote: post #10
posix_monad wrote: post #3

MLs require a lot of ceremony modelling simple record types. What we want to express here is an object with a map of properties (name to type): string Type map For the OOP minded: Map And also compose those: type Foo = { "_foo", int } type Bar = { "_bar", string } type FooBar = mergeMaps Foo Bar But at compile-time, of course. Have any languages achieved this? I know TypeScript can do some of these things, but it's c…

I think what you describe is called "extensible records". Elm had them in a prior version. You can implement them in a language with an expressive enough type system (e.g. Haskell or Scala) without special support, but the syntax can be a bit unwieldy. Here's three recent papers on extensible records: * https://arxiv.org/pdf/2108.06296 * https://arxiv.org/pdf/2404.00338 * https://dl.acm.org/doi/pdf/10.1145/3571224 I'…

> I think what you describe is called "extensible records".

From what I read, gp only asks for a way to express the type, not a way to change/extend the type of something during runtime.

Elm's current implementation does this just fine.

Re: Types as Interfaces

#16

Post by jiehong »

ivanbakel wrote: post #8
posix_monad wrote: post #3

MLs require a lot of ceremony modelling simple record types. What we want to express here is an object with a map of properties (name to type): string Type map For the OOP minded: Map And also compose those: type Foo = { "_foo", int } type Bar = { "_bar", string } type FooBar = mergeMaps Foo Bar But at compile-time, of course. Have any languages achieved this? I know TypeScript can do some of these things, but it's c…

This is often referred to as "row typing", and the only language I've ever used that implemented it first-class with polymorphism is Purescript[0]. It's possible to model such things in other languages with sufficiently powerful type systems, though they're normally not as nice to use. As an aside, Purescript is one of the most fun languages I've ever used for frontend work, and I lament that Elm seems to have overta…

Even if Elm has had no new version in 4 years while purescript has?

You made me want to try purescript :)

Re: Types as Interfaces

#17

Post by jcelerier »

posix_monad wrote: post #3

MLs require a lot of ceremony modelling simple record types. What we want to express here is an object with a map of properties (name to type): string Type map For the OOP minded: Map And also compose those: type Foo = { "_foo", int } type Bar = { "_bar", string } type FooBar = mergeMaps Foo Bar But at compile-time, of course. Have any languages achieved this? I know TypeScript can do some of these things, but it's c…

That sounds achievable at compile-time in c++ with some type-level map implementation such as in boost.mp11 (sadly the language does not allow general use of types as values preventing the use of standard map containers).

Re: Types as Interfaces

#19

Post by brabel »

posix_monad wrote: post #3

MLs require a lot of ceremony modelling simple record types. What we want to express here is an object with a map of properties (name to type): string Type map For the OOP minded: Map And also compose those: type Foo = { "_foo", int } type Bar = { "_bar", string } type FooBar = mergeMaps Foo Bar But at compile-time, of course. Have any languages achieved this? I know TypeScript can do some of these things, but it's c…

The Ceylon language allowed you to do that sort of thing with types. If you "added" two maps together of types `Map` and `Map`, you would get back a `Map` (or, equivalently, `Map | Map`).

But for some slightly complex reasons, most language designers find adhoc union types (which are required for this to work) a bad idea. See the Kotlin work related to that, they explicitly want to keep that out of the language (and I've seen that in other language discussions - notice how tricky it can be to decide if two types are "the same" or whether a type is a subtype of another in the presence of generalized unions) except for the case of errors: https://youtrack.jetbrains.com/issue/KT-68296/Union-Types-fo...

Re: Types as Interfaces

#20

Post by gpderetta »

posix_monad wrote: post #3

MLs require a lot of ceremony modelling simple record types. What we want to express here is an object with a map of properties (name to type): string Type map For the OOP minded: Map And also compose those: type Foo = { "_foo", int } type Bar = { "_bar", string } type FooBar = mergeMaps Foo Bar But at compile-time, of course. Have any languages achieved this? I know TypeScript can do some of these things, but it's c…

You can easily do composable tagged (by index type or even comp time strings) tuples in C++. The syntax is not pretty, but there is minimal to non-existent overhead.

On the other hand, thanks to the loose "duck typed" nature of templates, you can write functions that act on a subset of the fields of a type (even type checking it with concepts) while preserving the type of the whole object.

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