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Editorial Team
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Asked: 2026-06-02T00:44:55+00:00

UPDATE All the answers are good here, but @senia’s does so the most directly,

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UPDATE
All the answers are good here, but @senia’s does so the most directly, without need for additional steps. Will this lead to bugs, possibly, but when using Map[Symbol, T] convention in hundreds of methods, a 1-step implicit conversion prior to map creation is preferred (avoids Symbol Map key permgen storage). At any rate, here’s the pimp:

class SymbolProvidesPair(i: Symbol) { def ->[T](s: T) = (i.toString.tail, s) }
@inline implicit def symbol2String(i: Symbol) = new SymbolProvidesPair(i)

Original
It bothers me a bit using string keys in Maps, just slows me down and is, IMO, not as syntactically easy on the eyes as symbol keys.

val map: Map[String, Int] = Map("strings" -> 1, "blow" -> 2)
val map: Map[String, Int] = Map('symbols -> 1, 'rock -> 2)

So, I created an implicit to scratch my itch:

implicit def symbolKey2String[A <: Symbol, B](x:(A,B)) = (x._1.toString, x._2)

Couple things:
1) is this the correct signature? The above works, but A <: Symbol I take to mean, something that derives from Symbol vs. something that equals Symbol.

2) I’ll be using this when I manually type out Maps; i.e. just for convenience. Am I going to hit any snags with this implicit? It seems edge case enough to not cause issues (like string2Int, for example), but not sure if I’m missing something.

Thanks

EDIT
Ok, well #1 I can just actually say what I mean, [Symbol, B] instead of [A <: Symbol, B]

But now I find myself with another issue, the symbol-to-string implicit boxes me into a corner of sorts as I then have to explicitly define Map[String, Type] for all new Maps (i.e. lose the nice compiler type inference) in order to be able to use symbol keys.

How then to get the best of both worlds, Map symbol keys, but with inferred [String, Type] when not specifying the type signature? i.e. have the compiler infer Map[String, Int] when I do:

val map = Map('foo -> 1)
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1 Answer

  1. Editorial Team

    You don’t need to specify map’s type explicitly:

    scala> class SymbolToPait(i: Symbol) { def ->[T](s: T) = (i.toString().tail, s)}
defined class SymbolToPait

scala> implicit def symbolToPair(i: Symbol) = new SymbolToPait(i)
symbolToPair: (i: Symbol)SymbolToPait

scala> 'Symbol -> "String"
res0: (String, String) = (Symbol,String)

scala> Map('Symbol -> "String")
res1: scala.collection.immutable.Map[String,String] = Map(Symbol -> String)

scala> Map('Symbol -> 1)
res2: scala.collection.immutable.Map[String,Int] = Map(Symbol -> 1)

    This kind of behavior could surprise other developers. Maybe it would be better to replace -> with some other word? For example :-> or ~>.

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