How do I define a generic (polymorphic) function in Rascal?

Question

I would like to define a generic function to extract keys from a map, something like this:

public list[K] keys(map[K,V] aMap) {
    return [ key | key:val <- aMap ];
}

Although no syntax error is given, this does not work. Is there a way to do it?

Answer 1

You can define this as

public list[&K] keys(map[&K,&V] aMap) {
    return [ k | k <- aMap ];
}

Note that the keys are unordered, so it may make more sense to return them as a set instead of as a list. You can also always get the keys or values out as sets directly by projecting them out of the map, using either

aMap<0>

for the set of keys or

aMap<1>

for the set of values. Finally, the Set module contains a toList function, so you could do this in one line as

toList(aMap<0>)

which will give you the same result as calling the keys function.