Re: Type inferencing pattern matching anonymous functions

Replying to my own question, just to make sure the solution is archived.

On 1/26/07, Rafael de F. Ferreira <rafael@xxxxxxxxxxxxxxxxxx> wrote:
> I hit a problem when dealing with {case P => R ...} expressions and
> wanted to ask if anyone has found a better solution. I wrote the
> following helper function:
>   def replace[T](l:List[T], f:PartialFunction[T,T]) : List[T] = l match {
>      case Nil => Nil
>      case x::xs => (if (f.isDefinedAt(x)) f(x) else x) :: replace(xs, f);
>   }
>
> The idea is to call it with a {case ...} function as the second
> parameter like this:
>   case class Something(i:int)
>   val double1 = replace(original, {case Something(2) => Something(4)})
> //error here
>
> That didn't work because the compiler can't figure out the type of the
> function (a "missing parameter type" error is reported). If it is
> explicitly supplied to replace() then no compilation errors occur:
>   val double2 = replace[Something](original, {case Something(2) =>
> Something(4)}) //OK
>
> The best solution I could find to retain the generality without being
> too verbose is to curry the type argument before the call site:
>   def replSomething = &replace[Something];
>    ...
>   val double3 = replSomething(original, {case Something(2) => Something(4)})
>
> Has anyone got a better idea?
I do :-)

Inspired by Martin's message to the list explaining the foldLeft
syntax, I changed the method declaration to "explicitly" curry the
second parameter. This way the compiler is able to infer the correct
type of the anonymous function being passed. It ended up like this:

def replace[T](l:List[T])(f:PartialFunction[T,T]) : List[T] = l match {
 case Nil => Nil
 case x::xs => (if (f.isDefinedAt(x)) f(x) else x) :: replace(xs)(f);
}

case class Something(i:int)
val original = List(Something(1), Something(2), Something(3))
val double1 = replace(original){case Something(2) => Something(4)}

> Thanks a lot;
> --
> Rafael de F. Ferreira.
> http://www.rafaelferreira.net/

--
Rafael de F. Ferreira.
http://www.rafaelferreira.net/