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0 pointsKMag5y ago0 comments

This creates two types with quotes in their type names, and then creates a union type of the two? Then the compiler needs to determine if the token's context requires a type or a value, and disambiguate accordingly?

In the example above, the created type "Scala" is distinct from a type Scala, correct?

I did some quick poking around for Scala literal types, but I found stuff like https://github.com/jeremyrsmith/literal-types , which don't look like the example above.

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11 comments · 2 top-level

paavohtl5y ago· 6 in thread

Literal types quite simply means that values are types. For example in TypeScript the literal type "Foo" is a special string type, where the only valid value is the literal "Foo" – assigning "Bar" to a variable with the type "Foo" is a compile time type error. Literal types can be combined into union types like any other type.

cutler5y ago

Sounds like more Scala sophistry. I thought they were supposed to be aiming for pragmatism with Scala 3.

crucialfelix5y ago

I use it in Typescript often. Very useful.

type Verb = 'GET' | 'POST' | 'PUT' | 'DELETE';

export type TerminusType = 'ICAO' | 'IATA' | 'LOCODE' | 'ADDRESS' | 'LATLNG';

It's much better than just assuming it's a string and hoping you don't make a typo.

If the value is supplied from user input then it ensures that you write checks or switch cases.

1 more reply

doikor5y ago

This is just making clear and easy to use something that people did using some type level magic (see shapeless Witness).

Something that people used through ugly hacks becoming part of the language natively in a easy to use and understand (and faster to compile) way sounds very much like pragmatism to me.

Here is the source code to Witness (and other singleton stuff in shapeless) https://github.com/milessabin/shapeless/blob/main/core/src/m...

esarbe5y ago

I can think of plenty cases where I would want to limit the input space of a function to a known set of literals.

Just as an example; the `document.getContext` [0] api takes a string argument to select the context canvas.

Without literal types, you cannot enforce this to a set of valid values. Literal types gives you the power to tell the compiler, which values are valid.

Since there are other programming languages that support literal types, it's not just Scala sophistry. :-P

[0] https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasE...

nsonha5y ago

that's exactly how it works in TS. You can call TS sophistry then. Most people I talk to put it on the pragmatic side

mjburgess5y ago

They're describing typescript...

nicktelford5y ago· 3 in thread

No. It's a Singleton type, which means that instead of just being a String, it has to be exactly the String "Scala".

i.e.

val x: "scala" = "scala" // would compile

val y: "scala" = "java" // would produce a compile error

Union types are an orthogonal concept, and work as you'd expect:

val z: String | Int = "foo"

KMagOP5y ago

Interesting, and I presume it works for singletons of any type, correct?

    val x: 1 = 1
    val y: 2.718281828 = 2.718281828

That's kind of cute. I presume its main purpose is for overrides in order to get something similar to template specialization. C++ templates can be specialized by types and by values, but Scala method overrides are only by type, and literal types allow you to present a value as a type, so if the value can be statically proven to match, then the override is called?

paavohtl5y ago

In TypeScript perhaps the most important use case for literal types is the ability to to create discriminated unions, or algebraic datatypes (ADTs). Many modern languages have a separate feature for this (such as enums in Rust), but in TypeScript they can be constructed using literal and union types. Here's an example:

    interface ProductOffer { 
        kind: 'productOffer'
        eans: Ean[]
        discountPercentage: number
    }

    interface GroupOffer {
        kind: 'groupOffer'
        groups: GroupId[]
        discountPercentage: number
    }

    type Offer = ProductOffer | GroupOffer

Also check the TypeScript handbook for more information: https://www.typescriptlang.org/docs/handbook/2/narrowing.htm...

1 more reply

lmm5y ago

It's not really about specialization (types in Scala tend to be parametric most of the time) so much as just being able to have values in types, so that you can have types like "ListOfLength[1]" or "MapContainingKeys["Foo" :: "Bar" :: Nil]" without having to do some horrendous encoding of those things as types.

j / k navigate · click thread line to collapse

0 comments

11 comments · 2 top-level

paavohtl5y ago· 6 in thread

cutler5y ago

Sounds like more Scala sophistry. I thought they were supposed to be aiming for pragmatism with Scala 3.

crucialfelix5y ago

I use it in Typescript often. Very useful.

type Verb = 'GET' | 'POST' | 'PUT' | 'DELETE';

export type TerminusType = 'ICAO' | 'IATA' | 'LOCODE' | 'ADDRESS' | 'LATLNG';

It's much better than just assuming it's a string and hoping you don't make a typo.

If the value is supplied from user input then it ensures that you write checks or switch cases.

1 more reply

doikor5y ago

This is just making clear and easy to use something that people did using some type level magic (see shapeless Witness).

Something that people used through ugly hacks becoming part of the language natively in a easy to use and understand (and faster to compile) way sounds very much like pragmatism to me.

Here is the source code to Witness (and other singleton stuff in shapeless) https://github.com/milessabin/shapeless/blob/main/core/src/m...

esarbe5y ago

I can think of plenty cases where I would want to limit the input space of a function to a known set of literals.

Just as an example; the `document.getContext` [0] api takes a string argument to select the context canvas.

Without literal types, you cannot enforce this to a set of valid values. Literal types gives you the power to tell the compiler, which values are valid.

Since there are other programming languages that support literal types, it's not just Scala sophistry. :-P

[0] https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasE...

nsonha5y ago

that's exactly how it works in TS. You can call TS sophistry then. Most people I talk to put it on the pragmatic side

mjburgess5y ago

They're describing typescript...

nicktelford5y ago· 3 in thread

No. It's a Singleton type, which means that instead of just being a String, it has to be exactly the String "Scala".

i.e.

val x: "scala" = "scala" // would compile

val y: "scala" = "java" // would produce a compile error

Union types are an orthogonal concept, and work as you'd expect:

val z: String | Int = "foo"

KMagOP5y ago

Interesting, and I presume it works for singletons of any type, correct?

    val x: 1 = 1
    val y: 2.718281828 = 2.718281828

paavohtl5y ago

    interface ProductOffer { 
        kind: 'productOffer'
        eans: Ean[]
        discountPercentage: number
    }

    interface GroupOffer {
        kind: 'groupOffer'
        groups: GroupId[]
        discountPercentage: number
    }

    type Offer = ProductOffer | GroupOffer

Also check the TypeScript handbook for more information: https://www.typescriptlang.org/docs/handbook/2/narrowing.htm...

1 more reply

lmm5y ago

j / k navigate · click thread line to collapse