Scala parameterized methods and arithmetic operations

Question

I am trying to get a simple piece of functionality to work where I have a List of Lists and I want to do some mathematical operations on the data (-, + , *, /). I want the method to take any of the following types (Int, Float, Double).

here is what I have tried:

def doSomething[T](data: List[T]){
 data reduceLeft(_ / _)
}

the following is displayed: value / is not a member of type parameter T.

How do I get this to work for the AnyVal types (Double, Int, Float)?

Update I tried implementing the suggestion in the following code:

def dot[T](l: List[List[T]])(implicit num: Numeric[T]) = 
{

    for (row <- data)
        yield for(col <- l)
            yield row zip col map {a => num.times(a._1 , a._2)}   reduceLeft (_+_)

and get the error: type mismatch; found : a._1.type (with underlying type T) required: T

Is there any way to get around that?

i figured out one way to do this was to have it contained within a parameterized class. But, I am still wondering why the above doesn't work? Especially if I wanted to use this method as a Function Object. — Dan_Chamberlain
– Dan_Chamberlain, Commented Jan 18, 2011 at 23:25
It seems it doesn't work because times requires both arguments to be of the same type T, in this case there is no ensurance that data's underlying type T and dot's type T are the same type. If you want to avoid parameterized class, try passing to function data parameter as well — fehu
– fehu, Commented Jan 19, 2011 at 0:38
for example def dot[T](data: List[List[T]])(l: List[List[T]])(implicit num: Numeric[T]) = ..., then use dot(data) _ — fehu
– fehu, Commented Jan 19, 2011 at 0:52

fehu · Accepted Answer · 2011-01-17 03:38:18Z

9

For division:

def foo[T](l: List[T])(implicit num: Numeric[T]) = num match{
     case i: Integral[_] => l reduceLeft (i.quot(_, _))
     case fr: Fractional[_] => l reduceLeft (fr.div(_, _))}

For +, - and * it's easier (plus, minus, times respectively):

def foo[T](l: List[T])(implicit num: Numeric[T]) = l reduceLeft (num.plus(_, _))

edited Jan 17, 2011 at 3:38

answered Jan 17, 2011 at 3:21

fehu

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2 Comments

Landei Over a year ago

It should be mentioned that what you see here is often called the "(Implicit) Type Class Pattern". See e.g. java.dzone.com/articles/scala-implicits-type-classes

Dan_Chamberlain Over a year ago

Now the trouble appears to be with using tuple values with Numeric. I have updated my example to show the issue.

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