Rewrote unified types tour #708

travissarles · 2017-02-22T17:33:11Z

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Ichoran · 2017-02-22T18:01:12Z

tutorials/tour/_posts/2017-02-13-unified-types.md


-`scala.AnyVal` represents value classes. All value classes are non-nullable and predefined; they correspond to the primitive types of Java-like languages. Note that the diagram above also shows implicit conversions between the value classes.
+`scala.AnyVal` represents value classes. All value classes are non-nullable and predefined; they correspond to the primitive types of Java-like languages: `Byte`, `Short`, `Char`, `Int`, `Long`, `Float`, `Double`, and `Boolean`. Scala adds the `Unit` type which is similar to Java's `void`.


The automatic conversions between primitives is really important to know about and shouldn't be left out. Unit is not similar to void in a very important way which is that it is a value that you can return, unlike void which is a special case of not returning anything; if that isn't part of the tour by this point, you must explain it or "similar to void" is more misleading than helpful.

Ichoran · 2017-02-22T18:02:50Z

tutorials/tour/_posts/2017-02-13-unified-types.md


 ## Scala Class Hierarchy ##

-The superclass of all classes `scala.Any` has two direct subclasses: `scala.AnyVal` and `scala.AnyRef`.
+`scala.Any` is the base type of all types. It defines certain universal methods: `==`, `!=`, `equals`, `hashCode`, `toString`. `scala.Any` has two direct subclasses: `scala.AnyVal` and `scala.AnyRef`.


The list isn't complete. (See http://www.scala-lang.org/api/2.12.1/scala/Any.html). Either be complete, or give examples (and say they're examples).

Ichoran · 2017-02-22T18:07:51Z

tutorials/tour/unified-types-diagram.dot

@@ -0,0 +1,13 @@
+digraph UnifiedTypes {


This graph is misleading or incomplete in numerous ways, including

Makes it look like AnyRef has only three children

Makes it look like Null and Nothing are logically on the same level (they are not)

Does not show the subtyping relationship between List and Seq

Doesn't show conversions between primitives

As it stands the graph is worse than nothing. It's really important to get this stuff right to start people off with an accurate mental model instead of a bunch of misconceptions that have to be unlearned.

I agree that the graph is misleading; some ellipses or other indication of what's missing would be helpful.

I think the graph is still useful to give users a global picture of what is written in this page. But I agree with point 1. of @Ichoran.

Ichoran · 2017-02-22T18:47:36Z

tutorials/tour/_posts/2017-02-13-unified-types.md

@@ -11,15 +11,15 @@ next-page: classes
 previous-page: basics
 ---

-In Scala, all values are instances of a class, including numerical values and functions. The diagram below illustrates the class hierarchy.
+In Scala, all values are instances of a class, including numerical values and functions. The diagram below illustrates a subset the class hierarchy.


a subset of

tpolecat · 2017-02-22T18:38:26Z

tutorials/tour/_posts/2017-02-13-unified-types.md

+
+## Nothing and Null
+`Nothing` is a subtype of all types. There is no value that has type `Nothing`. There are two uses: The first is to signal abnormal termination such as an exception (i.e. it doesn't return). The second type usage is an element type of empty collections (e.g. `Set[Nothing]`).
+


A few suggested edits:

Nothing is a subtype of all types, also called the bottom type. There is no value that has type Nothing. There are two common uses: the first is to signal non-termination such as a thrown exception, program exit, or an infinite loop (i.e., it is the type of an expression which does not evaluate to a value, or a method that does not return normally); the second common usage is as the element type of empty collections (e.g. Set[Nothing]).

What about using “that does not terminate” instead of “that does not return”?

I think Set[Nothing] is probably not a good example because it happens that (even immutable) Set is invariant in Scala. You could use Nil (which has type List[Nothing]), alternatively.

I’m not sure we should mention this use case of Nothing here, though… You can not really explain it without explaining how does variance work.

I think we could say something like "we can conclude from the type List[Nothing] that Nil must be empty" which gets the point across while glossing over variance. This is also the case for Set[Nothing] of course, but as you noted it ends up not being useful due to invariance.

tpolecat · 2017-02-22T18:45:27Z

tutorials/tour/_posts/2017-02-13-unified-types.md

+## Nothing and Null
+`Nothing` is a subtype of all types. There is no value that has type `Nothing`. There are two uses: The first is to signal abnormal termination such as an exception (i.e. it doesn't return). The second type usage is an element type of empty collections (e.g. `Set[Nothing]`).
+
+`Null` is a subtype of all subtypes of `AnyRef`. The class name is `Null` but the literal identifier is `null`. You can assign any reference type to null.


A few suggested edits:

Null is a subtype of all reference types (i.e., subtypes of AnyRef). It has a single value identified by the keyword literal null. You can assign any reference type to null.

It might also be helpful to note that null is provided mostly for interoperability with other JVM languages and is not commonly used in Scala code.

I would even be comfortable with a stronger wording like "is never used in normal Scala code".

tpolecat · 2017-02-22T18:48:01Z

tutorials/tour/_posts/2017-02-13-unified-types.md

@@ -11,15 +11,15 @@ next-page: classes
 previous-page: basics
 ---

-In Scala, all values are instances of a class, including numerical values and functions. The diagram below illustrates the class hierarchy.
+In Scala, all values are instances of a class, including numerical values and functions. The diagram below illustrates a subset the class hierarchy.


I don't think it's useful to talk about classes here. I suggest

All Scala types inhabit a unified lattice with Any at the top and Nothing at the bottom.

After our discussion on gitter I think that indeed, talking about the type hierarchy is probably better.

already settled I guess, but I agree that it's better to say "type" instead of "class" in this (and many other) contexts

Related, the eventual diagram shouldn't include Seq for instance because that's not a type. Seq[Int] would be ok though.

Seq [...] that's not a type.

really? is that not just a pedantic objection? perhaps I misunderstand; can you justify how a beginner might actually be led astray, there?

what about Seq[_]? would that get rid of the bathwater without losing the baby?

I don't think it's pedantic … Seq is a type constructor, not a type. You can't have a variable of type Seq. You're right of course that we could say Seq[_] but that's shorthand for Seq[A] forSome { type A } which I think raises more questions than it answers.

meh... I don't agree this is sufficient reason to leave it out of the diagram. the distinction between type constructors and types isn't important here. I wouldn't object to a footnote with a link to a later section where the issue you raise is addressed.

fellow old man yells at other old man

tpolecat · 2017-02-22T18:49:05Z

tutorials/tour/_posts/2017-02-13-unified-types.md


 ## Scala Class Hierarchy ##

-The superclass of all classes `scala.Any` has two direct subclasses: `scala.AnyVal` and `scala.AnyRef`.
+`scala.Any` is the base type of all types. It defines certain universal methods: `==`, `!=`, `equals`, `hashCode`, `toString`. `scala.Any` has two direct subclasses: `scala.AnyVal` and `scala.AnyRef`.


For the first sentence I suggest

Any is the supertype of all types, also called the top type.

tpolecat · 2017-02-22T18:54:50Z

tutorials/tour/unified-types-diagram.dot

@@ -0,0 +1,13 @@
+digraph UnifiedTypes {


I agree that the graph is misleading; some ellipses or other indication of what's missing would be helpful.

travissarles · 2017-03-14T15:03:44Z

@heathermiller ready to merge

SethTisue · 2017-03-14T21:34:39Z

when I build the site locally I see:

looks like some malformed Markdown?

SethTisue · 2017-03-14T21:35:51Z

otherwise LGTM for merge

Ichoran reviewed Feb 22, 2017

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tpolecat reviewed Feb 22, 2017

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SethTisue mentioned this pull request Feb 23, 2017

ScalaObject no longer exists, but appears in diagram #615

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Rewrote unified types tour

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travissarles force-pushed the unified-types branch from 97288f2 to d5975e4 Compare February 24, 2017 14:46

travissarles mentioned this pull request Mar 17, 2017

Rewrite tour of Scala #733

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travissarles merged commit 7ac992c into scala:master May 10, 2017

SethTisue mentioned this pull request May 10, 2017

broken images on unified types page #769

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Rewrote unified types tour #708

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travissarles commented Mar 14, 2017

SethTisue commented Mar 14, 2017

SethTisue commented Mar 14, 2017


		`scala.AnyVal` represents value classes. All value classes are non-nullable and predefined; they correspond to the primitive types of Java-like languages. Note that the diagram above also shows implicit conversions between the value classes.
		`scala.AnyVal` represents value classes. All value classes are non-nullable and predefined; they correspond to the primitive types of Java-like languages: `Byte`, `Short`, `Char`, `Int`, `Long`, `Float`, `Double`, and `Boolean`. Scala adds the `Unit` type which is similar to Java's `void`.


		## Nothing and Null
		`Nothing` is a subtype of all types. There is no value that has type `Nothing`. There are two uses: The first is to signal abnormal termination such as an exception (i.e. it doesn't return). The second type usage is an element type of empty collections (e.g. `Set[Nothing]`).

Rewrote unified types tour #708

Rewrote unified types tour #708

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travissarles commented Feb 22, 2017

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SethTisue Feb 23, 2017 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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tpolecat Feb 23, 2017 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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travissarles commented Mar 14, 2017

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