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class Chain<T> {
constructor(public value: T) { }
then<S extends T>(cb: (x: T) => S): Chain<S> {
var t: T;
var s: S;
// Ok to go down the chain, but error to climb up the chain
(new Chain(t)).then(tt => s).then(ss => t);
// But error to try to climb up the chain
(new Chain(s)).then(ss => t);
// Staying at T or S should be fine
(new Chain(t)).then(tt => t).then(tt => t).then(tt => t);
(new Chain(s)).then(ss => s).then(ss => s).then(ss => s);
return null;
}
}
// Similar to above, but T is now constrained. Verify that the constraint is maintained across invocations
interface I {
x: number;
}
class Chain2<T extends I> {
constructor(public value: T) { }
then<S extends T>(cb: (x: T) => S): Chain2<S> {
var i: I;
var t: T;
var s: S;
// Ok to go down the chain, check the constraint at the end.
// Should get an error that we are assigning a string to a number
(new Chain2(i)).then(ii => t).then(tt => s).value.x = "";
// Staying at T or S should keep the constraint.
// Get an error when we assign a string to a number in both cases
(new Chain2(i)).then(ii => t).then(tt => t).then(tt => t).then(tt => t).value.x = "";
(new Chain2(i)).then(ii => s).then(ss => s).then(ss => s).then(ss => s).value.x = "";
return null;
}
}
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