FAQ for type inference

[som-snytt]

Documents scala/bug#7743

The tour still needs tweaking. per #2648

[SethTisue]

@som-snytt wdyt about the following version?

Why couldn't Scala infer the correct type for my code?

This is difficult to generalize about, as there could be a variety of causes and possible solutions.

Type inference was greatly improved in Scala 3. If you're using Scala 2, you might try your example in Scala 3 to see if it's handled any better.

The most common workaround is simply to add an explicit type to your code.

Sometimes using multiple parameter lists helps, as per this language tour section.

Questions about type inference often involve toSet. For discussion of examples involving toSet, see:

• List("ab").toSet.head.head won't compile bug#7743
• https://stackoverflow.com/questions/5544536/in-scala-2-type-inference-fails-on-set-made-with-toset

These discussions may shed light on other examples as well.

[SethTisue]

@BalmungSan this might interest you as well

[som-snytt]

If it's called dotty, then why doesn't my dotted expression work?

If I'm implying the correct type for my code, then why can't Scala infer it?

Are you using "imply" and "infer" correctly?

Expectation or Anticipation

I see my PR was more of a placeholder.

The github states are draft (WIP, drafts cannot be merged) and open green (ready for review), so if you're reviewing, it should be green. I don't know why there isn't a third state for "shared draft" that precedes green. That would mean, even if somebody accidentally grants an accepting review, don't merge it, as we're still mulling.

[BalmungSan]

Personally, I really like this FAQ addition.

My proposal for the title may be:

Why couldn't Scala infer the correct type for my dot-chain expression?

Additionally, I think providing a concrete example of the suggested solutions would be useful, as well as explaining why that fixes the problem.

Like:

// Chane this:
List("ab").toSet.head.head

// Into this:
val set = List("ab").toSet
set.head.head
// This works because ...

// Another option is:
List("ab").toSet[String].head.head
// This also works because ...

[SethTisue]

Additionally, I think providing a concrete example of the suggested solutions would be useful, as well as explaining why that fixes the problem

I'm consciously choosing not to do this. In general, I seek to keep FAQ entries short and sweet. A brief answer, and then a link or two or three to whatever other sources on the Internet (or on our own documentation site) exist that expound further.

[SethTisue]

That would mean, even if somebody accidentally grants an accepting review, don't merge it, as we're still mulling.

That was my intended meaning for marking it "draft", yeah.

[BalmungSan]

I'm consciously choosing not to do this. In general, I seek to keep FAQ entries short and sweet. A brief answer, and then a link or two or three to whatever other sources on the Internet

Oh right, forgot about that.
In that case I still expect whatever link is chosen to have such examples and explanations :)
Be it the official Scala documentation or a StackOverflow question / answer as other entries.

[som-snytt]

OK I will update with something like Seth's text. I'm having trouble with any title that says "dot". We say "dotted name" or similar for com.acme.foo, but "what do you call it when an expression uses a selection?" "Selection" seems too "specialized" a word for a FAQ. One reason this is a FAQ is that it's not obvious why I can't rewrite expressions in obvious ways and have it just work. You can say Liskov till you're blue in the face, referential transparency blah-blah, but if simple syntax tweaks break my code, then I will be confused and angry. I don't think a link to a ticket suffices, but a link to the Big Book of Scala can be supplied later.

[som-snytt]

ok I updated the section title.

toSet or not toSet, that is the question

I'm like, did I spell language right? It could be either affect or effect here, both work.

[SethTisue]

overall LGTM but I ask for one change: s/paradigmatic/common/

[som-snytt]

ok I accept pitching it to the commons

[som-snytt]

I neglected to mention that I inlined the links, to make it ASAP (short as preferred).

I changed paradigmatic question to stupid before the final edit.

I also changed toSet to toSeth.

[som-snytt]

It should say, "If you are here, you should ask BalmungSan".