value definitions before generator?

[fommil]

Any chance you could implement scala/bug#907 ?

[oleg-py]

The grammar of Scala defines for such that first generator is always an arrow
https://www.scala-lang.org/files/archive/spec/2.11/06-expressions.html#for-comprehensions-and-for-loops

So making it work nicely is not possible without actually modifying the syntax.

Some custom syntax can be made to work, however. E.g.:

for {
  x <- pure(42)
  y <- Option(33)
} yield x + y

This is syntactically valid, and after parser becomes:

pure(42).flatMap(((x) => Option(33).map(((y) => x))))

Which can be matched against and transformed into:

val x = 42 // parameter becomes a val
Option(33).map(((y) => x)) // lambda body of `flatMap` is inlined

This will save the user from needing to specify the monadic type they are operating on, as compared to 42.point[Option] and will not require pure to actually be available for the type.

[fommil]

"matched against and transformed into" ... and also that final .map could be removed if we're optimising for expansions 😄

pure is probably a bit of an overloaded term here, unless you're suggesting that this work for any definition of pure rather than a new one that you're proposing. I tend to use .pure[F] syntax in preference to pure(...)

[oleg-py]

pure is probably a bit of an overloaded term here, unless you're suggesting that this work for any definition of pure rather than a new one that you're proposing

The idea is that initial pure calls in for-comprehensions are handled by the plugin whereas after first non-pure expression nothing is changed.

I used pure just to illustrate what is actually possible without modifying parser. Would actually be happy if anybody suggests something better.

also that final .map could be removed if we're optimising for expansions 😄

Well, for Option(<constant>), sure. Though I doubt it is used often enough for anybody to actually benefit from :D.

[fommil]

what I mean is that if a for looks like

for {
  ...
  a <- doA
} yield a

the final .map is redundant and is just extra runtime object allocs.