如何将元组应用于Scala中的格式字符串？

Question

Edit1

I have already seen this question: Applying a function to a tuple in Scala

Ideally I would simply like to do like this:

scala> val t = ("A", "B", "C")
t: (java.lang.String, java.lang.String, java.lang.String) = (A,B,C)

scala> "%-10s %-50s %s".format(t) // or some closer syntax

Which should give output as

res12: String = A          B                                                  C

Edit2

Or in some sense Scala complier should be able to infer that I am actually calling with correct arguments and types such that

"%-10s %-50s %s".format(t.untuple) expands to "%-10s %-50s %s".format(t._1, t._2, t._3)

Can I use a macro to do this?

Original question follows

I have a tuple which I use for formatting a string:

scala> val t = ("A", "B", "C")
t: (java.lang.String, java.lang.String, java.lang.String) = (A,B,C)

scala> "%-10s %-50s %s".format(t.productElements.toList: _*)
warning: there were 1 deprecation warnings; re-run with -deprecation for details
res10: String = A          B                                                  C

scala> "%-10s %-50s %s".format(t._1, t._2, t._3)
res11: String = A          B                                                  C

All works fine till now. But this fails:

scala> val f = "%-10s %-50s %s".format(_)
f: Any* => String = <function1>

scala> f(t.productElements.toList: _*)
warning: there were 1 deprecation warnings; re-run with -deprecation for details
java.util.MissingFormatArgumentException: Format specifier '-50s'
    at java.util.Formatter.format(Formatter.java:2487)
    at java.util.Formatter.format(Formatter.java:2423)
    at java.lang.String.format(String.java:2797)
    at scala.collection.immutable.StringLike$class.format(StringLike.scala:270)
    at scala.collection.immutable.StringOps.format(StringOps.scala:31)
    at $anonfun$1.apply(<console>:7)
    at $anonfun$1.apply(<console>:7)
    at .<init>(<console>:10)
    at .<clinit>(<console>)
    at .<init>(<console>:11)
    at .<clinit>(<console>)
    at $print(<console>)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:704)
    at scala.tools.nsc.interpreter.IMain$Request$$anonfun$14.apply(IMain.scala:920)
    at scala.tools.nsc.interpreter.Line$$anonfun$1.apply$mcV$sp(Line.scala:43)
    at scala.tools.nsc.io.package$$anon$2.run(package.scala:25)
    at java.lang.Thread.run(Thread.java:744)

This also fails:

scala> f.apply(t)
java.util.MissingFormatArgumentException: Format specifier '-50s'
    at java.util.Formatter.format(Formatter.java:2487)
    at java.util.Formatter.format(Formatter.java:2423)
    at java.lang.String.format(String.java:2797)
    at scala.collection.immutable.StringLike$class.format(StringLike.scala:270)
    at scala.collection.immutable.StringOps.format(StringOps.scala:31)
    at $anonfun$1.apply(<console>:7)
    at $anonfun$1.apply(<console>:7)
    at .<init>(<console>:10)
    at .<clinit>(<console>)
    at .<init>(<console>:11)
    at .<clinit>(<console>)
    at $print(<console>)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:704)
    at scala.tools.nsc.interpreter.IMain$Request$$anonfun$14.apply(IMain.scala:920)
    at scala.tools.nsc.interpreter.Line$$anonfun$1.apply$mcV$sp(Line.scala:43)
    at scala.tools.nsc.io.package$$anon$2.run(package.scala:25)
    at java.lang.Thread.run(Thread.java:744)

What am I doing wrong ? How can I apply tuple parameters to a "varagrs" style function?

Answer 1

The problem is in lambda definition

val f = "%-10s %-50s %s".format(_)

is equivalent to:

val f = x => ("%-10s %-50s %s".format)(x) ,

So you're just passing sequence as first argument here.

Correct lambda is: (x: Seq[Any]) => ("%-10s %-50s %s".format)(x: _*) or even just val f: Seq[Any] => String = "%-10s %-50s %s".format

Examples:

scala> val f = x => ("%-10s %-50s %s".format)(x)
f: Seq[Any] => String = <function1>

scala> f(Seq(1,2,3))
java.util.MissingFormatArgumentException: Format specifier '-50s'

scala> val f: Seq[Any] => String = "%-10s %-50s %s".format
f: Seq[Any] => String = <function1>

scala> f(Seq(1,2,3))
res75: String = 1          2                                                  3

Unfortunately scala don't understand ("%-10s %-50s %s".format)(_: _*) , so you can't use underscore.

What's interesting here: if you try to do same using underscore without apply - you won't have to specify _* explicitly:

scala> val f = ("%-10s %-50s %s": scala.collection.immutable.StringLike[_]).format _
f: Seq[Any] => String = <function1>

scala> f(Seq(1,2,3))
res81: String = 1          2                                                  3

But you have to specify type class explicitly because function _ construction doesn't work with implicits .

Answer 2

You can define f as follows:

def f(t: Product) = "%-10s %-50s %s".format(t.productIterator.toList: _*)

then you can apply it to t :

scala> f(t)
res1: String = A          B                                                  C

What is Product

scala.Product is a trait which is implemented by many classes, for example Product2 and Product3 . These classes in turn are super types of Tuple2 and Tuple3 respectively.

So briefly, any tuple is a Product . That's why productIterator is available on all tuples.

So in order to use a tuple as vararg one way is to use the productIterator and convert it to List by invoking the toList method.

A better signature

Maybe the following signature is a better signature for your function:

 def f(fmt: String)(t:Product) = fmt(t.productIterator.toList: _*)

Now you can apply (or partially apply) it to get the desired result :

val res = f("%-10s %-50s %s")(t)
val newFormatter = f("%-10s %-40s %s")_
val newResult = newFormatter(t)