Missing Parameter Type Error

The built-in foreach function does exactly what you want:

Some("some").foreach( s => println(s))
None.foreach((s: String) =>println(s))

--j

On Wed, Jan 7, 2009 at 1:06 PM, Dave Ray <daveray@gmail.com> wrote:

Hello. I've only been using Scala about a week and am really enjoying
it. One confusing error I've gotten is related to this simple utility
function I created:

 def withSome[T](in : Option[T], code : (T) => Unit) {
   in match {
     case Some(s) => code(s)
     case None =>
   }
 }


Scala requires me to provide a type parameter even though it seems
like it should be able to infer it from the parameters. That is, if I
call:

  withSome(Some("some"), (s) => println(s))

I get "error: missing parameter type". However, if I provide the type parameter

  withSome[String](Some("some"), (s) => println(s))

everything works fine. What am I doing wrong?  Also, if there is a
built-in library function that provides the same functionality, or a
better way to do this, I'd love to hear about it. It's going to take
me a while to be able to figure out how to write idiomatic Scala code.

Thanks!

Dave

You can also use for-comprehensions to deal with options:

  for(s <- Some("some")) {
    println(s)
  }

Under the hood, this code translates to exactly the first line of my last email (that is, using 'foreach').

--j

On Wed, Jan 7, 2009 at 1:26 PM, Jorge Ortiz <jorge.ortiz@gmail.com> wrote:

The built-in foreach function does exactly what you want:

Some("some").foreach( s => println(s))
None.foreach((s: String) =>println(s))

--j

On Wed, Jan 7, 2009 at 1:06 PM, Dave Ray <daveray@gmail.com> wrote:

Hello. I've only been using Scala about a week and am really enjoying
it. One confusing error I've gotten is related to this simple utility
function I created:

 def withSome[T](in : Option[T], code : (T) => Unit) {
   in match {
     case Some(s) => code(s)
     case None =>
   }
 }


Scala requires me to provide a type parameter even though it seems
like it should be able to infer it from the parameters. That is, if I
call:

  withSome(Some("some"), (s) => println(s))

I get "error: missing parameter type". However, if I provide the type parameter

  withSome[String](Some("some"), (s) => println(s))

everything works fine. What am I doing wrong?  Also, if there is a
built-in library function that provides the same functionality, or a
better way to do this, I'd love to hear about it. It's going to take
me a while to be able to figure out how to write idiomatic Scala code.

Thanks!

Dave

To complement Jorge's answer, another way to call your function is to write:

withSome(Some("some"), (s: String) => println(s))

(adding the s: String type annotation)

Or writing the function with multiple parameter lists such that type inference applies only to the first parameter:

def withSome[T](in : Option[T])(code: T => Unit) {
  in match {
    case Some(s) => code(s)
    case None =>
    }
}

then you can simply write,

withSome(Some("some")) { s => println(s) }

alex


On Wed, Jan 7, 2009 at 11:06 AM, Dave Ray <daveray@gmail.com> wrote:

Hello. I've only been using Scala about a week and am really enjoying
it. One confusing error I've gotten is related to this simple utility
function I created:

 def withSome[T](in : Option[T], code : (T) => Unit) {
   in match {
     case Some(s) => code(s)
     case None =>
   }
 }


Scala requires me to provide a type parameter even though it seems
like it should be able to infer it from the parameters. That is, if I
call:

  withSome(Some("some"), (s) => println(s))

I get "error: missing parameter type". However, if I provide the type parameter

  withSome[String](Some("some"), (s) => println(s))

everything works fine. What am I doing wrong?  Also, if there is a
built-in library function that provides the same functionality, or a
better way to do this, I'd love to hear about it. It's going to take
me a while to be able to figure out how to write idiomatic Scala code.

Thanks!

Dave

As others have said, foreach does the same job.  But to answer your question, that behavior is irritating and perhaps strange, but there are workarounds.

scala> def withSome[T](in : Option[T], code : T => Unit) {
     |    in match {
     |      case Some(s) => code(s)
     |      case None =>
     |    }
     | }
withSome: [T](Option[T],(T) => Unit)Unit

scala> withSome(Some("some"), {s => println(s)})
<console>:6: error: missing parameter type
       withSome(Some("some"), {s => println(s)})
                               ^

If you use a naked function instead of a lambda, it Just Works <tm>

scala> withSome(Some("some"), println)
some

It also Just Works<tm> when you write the function in curried form

scala> def withSomeCurried[T](in : Option[T])(code : T => Unit) {
     |    in match {
     |      case Some(s) => code(s)
     |      case None =>
     |    }
     | }
withSomeCurried: [T](Option[T])((T) => Unit)Unit

scala> withSomeCurried(Some("some")){s => println(s)}
some

On Wed, Jan 7, 2009 at 11:06 AM, Dave Ray <daveray@gmail.com> wrote:

Hello. I've only been using Scala about a week and am really enjoying
it. One confusing error I've gotten is related to this simple utility
function I created:

 def withSome[T](in : Option[T], code : (T) => Unit) {
   in match {
     case Some(s) => code(s)
     case None =>
   }
 }


Scala requires me to provide a type parameter even though it seems
like it should be able to infer it from the parameters. That is, if I
call:

  withSome(Some("some"), (s) => println(s))

I get "error: missing parameter type". However, if I provide the type parameter

  withSome[String](Some("some"), (s) => println(s))

everything works fine. What am I doing wrong?  Also, if there is a
built-in library function that provides the same functionality, or a
better way to do this, I'd love to hear about it. It's going to take
me a while to be able to figure out how to write idiomatic Scala code.

Thanks!

Dave