June 23, 2015

# Agenda

In the next five minutes, I aim to:

• Demonstrate costs and benefits of TypeScript
• Argue that TypeScript is worth the investment

# Types crypt

The fundamental problem addressed by a type theory is to ensure that programs have meaning.

-- Mark Manasse

# Identity crisis

What does this function do?

``````function foo(x) {
// implementation hidden
}``````

# Parametricity

What does this function do?

``````function foo<A>(x: A): A {
// implementation hidden
}``````

# Possibilities Possibility

There's only one possible implementation*: `identity`.

``````function foo<A>(x: A): A {
return x;
}``````
* If we ignore `null` and `undefined`.

# Numeric product?

``var z = multiply(6, 7);``

What is `z`?

``function multiply(x, y) { return x * y; } // 42``
``function multiply(x, y) { return x + y; } // 13``
``function multiply(x, y) { x * y; } // undefined``
``function multiply(x, y) { return x; } // 6``
``function multiply(x, y) { return 'lol'; } // 'lol'``
``function multiply(x, y) { } // undefined``
``function multiply(x, y) { return arguments; } // undefined``

╯°□°）╯︵ ┻━┻

# Numeric product...

``var z: number = multiply(6, 7);``

Now what is `z`?

``function multiply(x: number, y: number): number { return x * y; } // 42``
``function multiply(x: number, y: number): number { return x + y; } // 13``
``// function multiply(x: number, y: number): number { x * y; }``
``function multiply(x: number, y: number): number { return x; } // 6``
``// function multiply(x: number, y: number): number { return 'lol'; }``
``function multiply(x: number, y: number): number { return arguments; } // undefined``
``// function multiply(x: number, y: number): number { return 'lol'; }``

┬─┬ノ( º _ ºノ)

# Numeric product!

``````interface Prod<A> {
product(x: A, y: A): A;
}

function product<A>(p: Prod<A>): (x: A, y: A) => A {
return p.product;
}

var numberProd: Prod<number> = {
product: (x: number, y: number) => { return x * y },
};``````
``var multiply: (x: number, y: number) => number = product(numberProd);``
``var z: number = multiply(6, 7); // 42``

\o/

# It's catchy

Web browsers don't know what TypeScript is. They need JavaScript.

``\$ tsc multiply.ts``

multiply.js:

``````var numberProd = {
product: function (x, y) { return x * y; }
};
function product(p) {
return p.product;
}
var multiply = product(numberProd);``````