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Type Constraints

rascal-0.34.0

Synopsis​

Type constraints restrict the acceptable type for parameters.

Syntax​

& Name <: Type

Description​

Constraints can be imposed on the actual types to which a type parameter may be bound. This is expressed by a subtype constraint which expresses that actual types bound to Name should be a subtype of Type.

Examples​

Here is the definition of the absolute value function abs from the Number library:

public &T <: num abs(&T <: num N)
{
    return N >= 0 ? N : -N;
}

The argument N is constrained to be at most of type num.

rascal>import util::Math;
ok
rascal>abs(-3);
int: 3
rascal>abs(-3.5);
real: 3.5

Here is an example from the Node library:

&T <: node setAnnotations(&T <: node x, map[str, value] annotations);

(we don't give the body of this function since it has been implemented in Java). setAnnotations takes a value of any type that is at most node and adds annotations to it. This makes it possible to set annotations on any Algebraic Data Type.

rascal>import Node;
ok
rascal>nd = "f"(10, "abc");
node: "f"(10,"abc")

First we apply setAnnotations to a node value:

rascal>setAnnotations(nd, ("color" : "red", "size" : "large"));
node: "f"(10,"abc",
  size="large",
  color="red")

Next, we introduce a new data type Suite:

rascal>data Suite = hearts() | diamonds() | clubs() | spades();
ok
rascal>st = diamonds();
Suite: diamonds()

And apply setAnnotations to a value of type Suite:

rascal>setAnnotations(st, ("player" : "Hossein", "gain" : "120"));
Suite: diamonds(player="Hossein",gain="120")