Tuple2 and maps

[Lorenz Bateman]

I've been reading the scala eBook and found an example of adding a key value pair to a map:

map += (1 -> "Some String")

it seems that the (1 -> "Some String") results in a Tuple2, however the (1, "Some String") also results in a Tuple2, as follows:

scala> 1 -> "Some String"
res8: (Int, java.lang.String) = (1,Some String)

scala> res8 getClass
res10: java.lang.Class[_] = class scala.Tuple2

scala> (1, "Some String")
res11: (Int, java.lang.String) = (1,Some String)

scala> res11 getClass
res12: java.lang.Class[_] = class scala.Tuple2

The problem I have is understanding why map += (1 -> "Some String") works fine, but:

scala> map += (1, "Some String")
<console>:6: error: type mismatch;
 found   : Int(1)
 required: (Int, String)
       map += (1, "Some String")
               ^

gives me an error.

Thanks

Lorenz

[Ricky Clarkson]

map += foo is shorthand for:

map.+=(foo)

Thus,

map += (1, "Some String") is an attempt to call:

map.+=(1, "Some String")

map has no method that accepts an Int and a String, though it does
have a method that accepts a tuple of Int and String.

Yes, it's sad that the two mechanisms are not interchangeable.

2008/10/8 Lorenz Bateman <lorenzb@...>:

> I've been reading the scala eBook and found an example of adding a key value
> pair to a map:
>
> map += (1 -> "Some String")
>
> it seems that the (1 -> "Some String") results in a Tuple2, however the (1,
> "Some String") also results in a Tuple2, as follows:
>
> scala> 1 -> "Some String"
> res8: (Int, java.lang.String) = (1,Some String)
>
> scala> res8 getClass
> res10: java.lang.Class[_] = class scala.Tuple2
>
> scala> (1, "Some String")
> res11: (Int, java.lang.String) = (1,Some String)
>
> scala> res11 getClass
> res12: java.lang.Class[_] = class scala.Tuple2
>
> The problem I have is understanding why map += (1 -> "Some String") works
> fine, but:
>
> scala> map += (1, "Some String")
> <console>:6: error: type mismatch;
>  found   : Int(1)
>  required: (Int, String)
>        map += (1, "Some String")
>                ^
>
> gives me an error.
>
> Thanks
>
> Lorenz
>

[Florian Hars-3]

Lorenz Bateman schrieb:
> The problem I have is understanding why map += (1 -> "Some String")
> works fine, but:
> scala> map += (1, "Some String")
> gives me an error.

Sometimes scala is a bit too clever in its handling of parens and you just have
to add another pair, in this case to disambiguate between these two methods:

def += (kv1 : (A, B), kv2 : (A, B), kvs : (A, B)*) : Unit
  Add two or more key/value pairs to this map.
  (The one you called)
def += (kv : (A, B)) : Unit
  Add a key/value pair to this map.
  (The one you thought you called)

Related:
http://www.nabble.com/-scala--Parenthesis-inference-td19535942.html
https://lampsvn.epfl.ch/trac/scala/ticket/1337

- Florian