composable-type-guards.ts

type Checker<T> = (x: any) => x is T;

function tArray<T>(f: Checker<T>) {
    return function (a:any): a is Array<T> {
        if (!Array.isArray(a)) return false;
        for (var k = 0; k < a.length; ++k)
            if (!f(a[k])) return false;
        return true;
    }
}

function tString(t: any): t is string { return typeof t === 'string' }

function tBoolean(t: any): t is boolean { return typeof t === 'boolean' }

function tObject<T>(checker: { [P in keyof T]: Checker<T[P]> }) {
    return function(obj: any): obj is T {
        for (var key in checker) if (!checker[key](obj[key])) return false;
        return true;
    }
}

var isTodoList = tObject({
    items: tArray(tObject({
        text: tString,
        completed: tBoolean
    })),
    showCompleted: tBoolean
});

function test(x: any) {
    if (isTodoList(x)) {
        x; // { items: { text: string, completed: boolean }[], showCompleted: boolean }
    }
}
 :tada: 4