5 ms·

Phantom Types in Gleam

35 points by pd-andy 6y ago

rq1 6y ago
This is a good post and technique in general. I used to use it extensively in C++: beware though of (super long) compile errors while nesting them through templates params. :) Edit: I was wrapping pointers for instance to not mix GPU and CPU memory addresses, to define a unidirectional flow of memory transfers between devices (and avoid manual synchronisation and tracking)... etc. You could somehow define a state machine through types and ensure compositional coherence.
- pd-andy 6y ago
  Using phantom types in state machine transitions is really cool, yeah. I came across a little gist that explored that idea in Elm [1] the other day, actually. [1]: https://gist.github.com/rupertlssmith/88946c8d207d7ad64daf4360fef1ac42 https://gist.github.com/rupertlssmith/88946c8d207d7ad64daf43...
Vanit 6y ago
It's kind of amusing reading this as a Typescript dev because I'm so used to Typescript's expressiveness I couldn't really work out which trick it was trying to tell me was novel. The answer is structural typing (aka duck typing).
- pd-andy 6y ago
  Not quite. I guess in some ways phantom types achieve the opposite of structural typing. In a language with structural typing, two types are considered equivalent if they share the same structure. With phantom types we can have a single structure and disambiguate between different uses without touching the underling structure. There's no difference between Id(User) and Id(Post) at runtime, the annotation is purely a compile-time restraint. In fact, because of typescript's structural typing, the only way to use phantom types in ts is to have some dummy field of type `never` that the type system can use to disambiguate between the two [1]. [1]: https://gist.github.com/GoNZooo/243b23702df1fae38c966ae18832c690 https://gist.github.com/GoNZooo/243b23702df1fae38c966ae18832...
  - mushishi 6y ago
    Holy smokes, that would be irritating :D Btw. is there a difference between these two: (s: string): PlainText => <PlainText><any>s; (s: string): PlainText => s as any as PlainText; Syntax-wise I'd rather use the latter for it's more clear in typed JSX-context.
    - pd-andy 6y ago
      As far as I know those two are equivalent. Definitely slightly clunkier, but is what it is.
  - timdeve 6y ago
    Yes, exactly, it's because of Structural Typing that you have to implement Phantom Types in that hacky way rather than Structural Typing being the "trick".
- deleted 6y ago
  [deleted]
muglug 6y ago
Amusing to see "even PHP" in there, as the author of the static analysis tool that makes that possible.
daniel5151 6y ago
I know it's a toy example, but it's a _really bad idea_ to use a float as a backing type for currency. https://stackoverflow.com/questions/3730019/why-not-use-double-or-float-to-represent-currency https://stackoverflow.com/questions/3730019/why-not-use-doub...
- pd-andy 6y ago
  Ha yes, very true!
- heavenlyblue 6y ago
  Have you worked in finance?

35 points by pd-andy 6y ago

rq1 6y ago

This is a good post and technique in general. I used to use it extensively in C++: beware though of (super long) compile errors while nesting them through templates params. :) Edit: I was wrapping pointers for instance to not mix GPU and CPU memory addresses, to define a unidirectional flow of memory transfers between devices (and avoid manual synchronisation and tracking)... etc. You could somehow define a state machine through types and ensure compositional coherence.

pd-andy 6y ago
Using phantom types in state machine transitions is really cool, yeah. I came across a little gist that explored that idea in Elm [1] the other day, actually. [1]: https://gist.github.com/rupertlssmith/88946c8d207d7ad64daf4360fef1ac42 https://gist.github.com/rupertlssmith/88946c8d207d7ad64daf43...

Vanit 6y ago

It's kind of amusing reading this as a Typescript dev because I'm so used to Typescript's expressiveness I couldn't really work out which trick it was trying to tell me was novel. The answer is structural typing (aka duck typing).

pd-andy 6y ago
Not quite. I guess in some ways phantom types achieve the opposite of structural typing. In a language with structural typing, two types are considered equivalent if they share the same structure. With phantom types we can have a single structure and disambiguate between different uses without touching the underling structure. There's no difference between Id(User) and Id(Post) at runtime, the annotation is purely a compile-time restraint. In fact, because of typescript's structural typing, the only way to use phantom types in ts is to have some dummy field of type `never` that the type system can use to disambiguate between the two [1]. [1]: https://gist.github.com/GoNZooo/243b23702df1fae38c966ae18832c690 https://gist.github.com/GoNZooo/243b23702df1fae38c966ae18832...
- mushishi 6y ago
  Holy smokes, that would be irritating :D Btw. is there a difference between these two: (s: string): PlainText => <PlainText><any>s; (s: string): PlainText => s as any as PlainText; Syntax-wise I'd rather use the latter for it's more clear in typed JSX-context.
  - pd-andy 6y ago
    As far as I know those two are equivalent. Definitely slightly clunkier, but is what it is.
- timdeve 6y ago
  Yes, exactly, it's because of Structural Typing that you have to implement Phantom Types in that hacky way rather than Structural Typing being the "trick".
deleted 6y ago
[deleted]

muglug 6y ago

Amusing to see "even PHP" in there, as the author of the static analysis tool that makes that possible.

daniel5151 6y ago

I know it's a toy example, but it's a _really bad idea_ to use a float as a backing type for currency. https://stackoverflow.com/questions/3730019/why-not-use-double-or-float-to-represent-currency https://stackoverflow.com/questions/3730019/why-not-use-doub...

pd-andy 6y ago
Ha yes, very true!
heavenlyblue 6y ago
Have you worked in finance?