How can I see the types inferred by the Scala compiler for expressions etc.? I have some code with complicated type inference and implicit conversions, and it's hard to see what's going on just by reading the code.
I've tried adding
scalacOptions in Compile += "-Xprint-types"
in build.sbt, but this has no effect.
Using scalac directly isn't very appealing because I have lots of dependencies.
I use the Eclipse Scala plugin and ENSIME to write code, and SBT to build.
It needs to be
scalacOptions in Compile ++= Seq("-Xprint-types", "-Xprint:typer")
instead.
Unfortunately the output isn't very readable. :(
This exact feature has been added in Eclipse Scala IDE 3.0!
Select any portion of code and press Ctrl-Shift-W T (replacing Ctrl by Cmd on Mac) to see the inferred type.
Hoist the expression to a non-local def or val, without an explicit type - then it will appear in the Outline view in Eclipse, with an inferred type assigned.
However, this isn't an ideal solution because it requires some work, and it can't be used when recursion is involved.
Related
We have problems when using Scalac -Xfatal-warnings in the following cases:
Implicit vals used by macros internally
Internal vals that macros auto-generate
In both cases, we see Scalac failing to compile because it detects some values are not used, while we know they are (simply, when we remove them, the code doesn't compile anymore)
Although the two might be symptoms of the same problem in Scalac, they boil down to the same issue for us: we need to disable the -Ywarn-unused in Scala 2.11.12
Is there a way to exclude specific class files so they won't be affected by the compiler flag?
As far as I know there is no way of disabling scalac flag for just one file (if you compile your whole project at once by e.g sbt). You can extract class into separate module with different compile flags.
In case of implicit vals used internally in macros, personally I use -Ywarn-macros:after flag, which make these implicits used in macro count as used. (Talking about Scala 2.12.4).
In Intellij IDEA, I have a Scala case class expecting Longs as arguments. The arguments passed in are numbers, which intellij interprets as Ints and reports an error on it. The code compiles without errors or warnings, so he compiler (and I) don't care. It's pretty easy to fix by putting a l after the number, but that is cumbersome and does not increase readability. Is there a setting somewhere in Intellij Idea to fix this behavior?
There is an open bug (see also the linked issues):
SCL-7729 Highlight error with implicit conversion of covariant type
I see this <<= symbol in lot of SBT code, but I don't what it does.
I tried googling for this symbol as well but I did not get any answers.
Can you please point me to some documentation or an example which clearly explains what does this symbol mean and what does it do?
Further to pfn's comment, this is described in the 0.12 docs under More Kinds of Settings. I guess it was dropped from the 0.13 docs because the same behaviour can now be defined in terms of :=.
Oh, the deep explanation is quite complicated.
Basically, the signature is:
def <<= (app: Initialize[Task[S]]): Setting[Task[S]] = macro std.TaskMacro.itaskAssignPosition[S]
So it involves this macro:
/* Implementations of <<= macro variations for tasks and settings. These just get the source position of the call site.*/
def itaskAssignPosition[T: c.WeakTypeTag](c: Context)(app: c.Expr[Initialize[Task[T]]]): c.Expr[Setting[Task[T]]] =
settingAssignPosition(c)(app)
I already used this kind of operator when dealing with AspectJ compilation:
products in Compile <<= products in Aspectj
Basically, it means: base the code source on the AspectJ source files (generated with a plugin), not the classical ones.
I interpret it as a kind of "replaceAll/erase":
Replace bunch of files to compile by the files involving AspectJ annotations.
I am using slick's for expression, and seeing some strangeness in terms of intellij's inferred type. Here are the unexpected result:
Why in the second case, it is not Query[Int, Int] but instead becomes Query[Nothing, Nothing]. I seem to lose some type information.
What's to say? IDEA has it's own parser and typer and it is not that hard to confuse it. If you care about fidelity between your IDE and the Scala compiler, the Scala IDE (which runs in Eclipse) would be better for you.
Personally, I dislike Eclipse more than I do the vagaries of the IDEA Scala plug-in.
According to "Programming in Scala" one can pass the argument -Xprint:typer to the compiler and gets the code back as it looks after all the implicites are actually applied.
I also found that I can set compiler arguments in the project properties.
But I can't find any resulting output anywhere ...
So where do I have to look?
If you start Eclipse from a console, you should see the printed output there.
With the Scala IDE 2.1 Milestone 1 you can press Ctrl-1 to make implicits explicit
From http://scala-ide.org/download/milestone.html#scala_ide_21_milestone_1
Highlight Implicits It has never been easier to know where implicits
are applied. And, by pressing Cmd/Ctrl+1, turn an implicit conversion
into an explicit call!