Add doc page for Literal Singleton Types #2699

Varunram · 2017-06-07T06:40:55Z

Intended as a fix for #2615.

My first time writing a doc page, so please do comment your views. I do feel that some prose can be added, which I'm searching for. CC @OlivierBlanvillain

lforite · 2017-06-07T07:27:13Z

docs/docs/reference/singleton-types.md

+Right now, a popular way to replicate literal singleton types is to create macros and/or use libraries such as
+`shapeless` due to the lack of a syntax to express them.
+
+With Literal Singelton types, it is possible to do something like


it seems there is a typo on Singelton

Oh, no. Really sorry, will edit that. Thanks!

lforite · 2017-06-07T07:29:52Z

docs/docs/reference/singleton-types.md

+val t: 42 = 42
+```
+
+to use literal Singleton types. Trying to do the same in Scala produces the following error:


I think to use literal Singleton types is redundant with With Literal Singelton types. What do you think ?

OlivierBlanvillain

Thanks for starting this up! I wouldn't mind having a very short doc here, just explaining what they are and how to use them. It might be nice to add examples with functions taking / returning singleton types such as the one I pointed in comments, or also this one:

def f(t: Double): t.type = t

val a: 1.2 = f(1.2)

OlivierBlanvillain · 2017-06-07T07:19:19Z

docs/docs/reference/singleton-types.md

+title: "Literal Singleton Types"
+---
+
+Singleton types is one of the most popular features in Scala. Literal Singleton types takes it a step further.


I would cut the intro down to the most important bit, something like this:

Literal Singleton Types allows primitive literals to be used as types. For example:

val fourtyTwo: 42 = 42 val hero: "Jedi".type = "Jedi" val trivial: true = true

OlivierBlanvillain · 2017-06-07T07:19:51Z

docs/docs/reference/singleton-types.md

+
+to use literal Singleton types. Trying to do the same in Scala produces the following error:
+```
+<console>:1: error: identifier expected but string literal found.


I don't see the point of showing this error

Will remove

OlivierBlanvillain · 2017-06-07T07:20:43Z

docs/docs/reference/singleton-types.md

+val t: "Jedi".type = "Jedi"
+```
+
+Dotty provides support for using Literal Singleton types of the last format namely:


This is Dotty's documentation, just show the Dotty format / example :)

OlivierBlanvillain · 2017-06-07T07:22:21Z

docs/docs/reference/singleton-types.md

+```
+and so on.
+
+There is a possibility to use singleton types in some contexts, but only on identifiers which points to a constant that conforms to `AnyRef`. This restriction is due to `Any` not having an eq method, which is what’s used for singleton type-equality check and pattern matching.


I'm assuming you are talking about type contexts here? I'm not aware of this restriction with AnyRef, do you have an example? This seems to work fine:

scala> def f[T <: Any](t: T) = t def f[T <: Any](t: T): [T](t: T)T scala> f[1.1](1.1) val res3: Double = 1.1

Ah, this one is on me again, I referred to some old documentation and was led to believe that. Thanks for pointing it out!

OlivierBlanvillain · 2017-06-07T07:26:33Z

docs/docs/reference/singleton-types.md

+---
+
+Singleton types is one of the most popular features in Scala. Literal Singleton types takes it a step further.
+Right now, a popular way to replicate literal singleton types is to create macros and/or use libraries such as


Right now

Doc should be timeless when possible.

If you want to cite shapeless here please do it more precisely by pointing to the corresponding documentation.

Sure, will edit the md by adding a link to shapeless. I thought of referencing this primarily because it was cited so in the SIP

lforite · 2017-06-07T07:31:04Z

docs/docs/reference/singleton-types.md

+```
+
+to use literal Singleton types. Trying to do the same in Scala produces the following error:
+```


Isn't this block missing scala syntax coloring ? I am not sure if it should be applied here as the first line is not Scala code

I guess I'll remove that part since its not needed :)

Varunram · 2017-06-07T10:23:20Z

Done! Thanks a ton @OlivierBlanvillain and @lforite for being so patient and pointing out changes!

OlivierBlanvillain · 2017-06-07T10:39:38Z

docs/docs/reference/singleton-types.md

+
+```scala
+val t: "Jedi".type = "Jedi"
+val t: 42.type = 42


These ones with a .type are not valid in Dotty

Um, that's why I had a separate section for Dotty earlier. One minute, will revert changes and push again.

Varunram · 2017-06-07T10:46:47Z

Okay, that wasn't supposed to happen. Lemme figure out why

Varunram · 2017-06-07T10:49:39Z

Really sorry for the delay! Some weird thing happened while resetting and that caused the PR to close. Thank you very much for putting up with me :)

felixmulder · 2017-06-07T11:15:54Z

docs/docs/reference/singleton-types.md

+
+```scala
+val t: 42 = 42
+val t: "Jedi" = "Jedi"


Maybe a different name for the second variable? :)

Varunram · 2017-06-10T04:53:51Z

ping @felixmulder @OlivierBlanvillain

OlivierBlanvillain · 2017-06-10T07:54:41Z

LGTM, thanks!

lforite reviewed Jun 7, 2017

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Varunram closed this Jun 7, 2017

Varunram force-pushed the singleton-types branch from 81522a4 to a527f3b Compare June 7, 2017 10:43

Varunram added 2 commits June 7, 2017 16:17

Create new doc page for Literal Singleton Types

9914606

Added Sidebar Entry for Singleton Types

491b2fa

Varunram reopened this Jun 7, 2017

felixmulder reviewed Jun 7, 2017

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Change name of variable

3d0abce

OlivierBlanvillain merged commit b38fa72 into scala:master Jun 10, 2017

SethTisue mentioned this pull request Dec 6, 2017

Document literal singleton types #2615

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Add doc page for Literal Singleton Types #2699

Add doc page for Literal Singleton Types #2699

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Varunram commented Jun 7, 2017

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Varunram commented Jun 7, 2017

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Varunram commented Jun 7, 2017

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Varunram commented Jun 10, 2017

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lforite Jun 7, 2017 •

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