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ThomasH
ThomasH

Reputation: 23516

Rascal: Can a Function return a Function

The Rascal documentation has an example of a function that takes a function as an argument:

int f(int x, int (int) multi){ return multi(x); }

Conversely, what is the syntax for a function that returns a function?

I couldn't find an example and tried things along the line:

(int (int)) f() {return (int y) {return y;}}

but got syntax errors in the repl.

Upvotes: 1

Views: 107

Answers (2)

Paul Klint
Paul Klint

Reputation: 1406

Here is an example:

int two(int n) = 2 * n;
int(int) g() = two;

Function two multiplies by 2 and g returns the function two. Observe that the return type of g is int(int), a type describing a function which returns an int and has one int argument.

A similar effect can be achieved by an inline definition:

int(int) g() = int(int n) { return 2 * n; };

You can also use this same notation inside other functions. For instance, you could create a function which multiplies two numbers:

int mult(int n, int m) = n * m;

If you use it, you would get what you would expect:

rascal>mult(3,4);
int: 12

You can instead return a function that essentially partially applies this function as follows:

int(int) multBy(int n) { 
    return int(int m) { 
        return mult(n,m); 
    }; 
}
int (int) (int)

So, this returns a function that takes an int and returns an int (int), i.e., a function that takes an int and returns an int. You can then use it as so:

rascal>multBy3 = multBy(3);
int (int)

rascal>multBy3(4);
int: 12

You can find more examples in some of our (many) files with tests:

  • lang::rascal::tests::basic::Functions
  • lang::rascal::tests::functionality::FunctionComposition

Thanks for your question, we have more documentation to do!

Upvotes: 2

ThomasH
ThomasH

Reputation: 23516

The short answer to my failed attempt is:

  • leave out the outermost parens in the return type of f
  • add the return type int of the anonymous function that is returned by f
  • don't forget the semi after f's return statement

That gives:

int (int) f() { return int (int y) { return y; } ; }

Upvotes: 1

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