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A truly strongly-typed printf in TypeScript

With template literal types in TypeScript, we can make our own mini language, such as a strongly typed version of printf, that can be called as,

printf('I am %s, %d years old, born in %D', arg1, arg2, arg3);

Constructing, destructuring symmetry, and pattern matching

Ideally we want symmetric constructing and destructuring, or, something can be destructured the same way it's constructed.

This is actually true most of the time for values, for example maps const { foo, bar } = { foo: 'foo', bar: 1 }, or arrays / tuples const [foo, bar] = ['foo', 'bar'].

It should come as a pleasant surprise that such destructuring works equally well on type level. The trick is to use type inference (extends + infer) to destructure or pattern match.

type FooType<T> = T extends { foo: infer F } ? F : never;

// must have number type
const fieldFoo: FooType<{ foo: number }> = 123;

Pattern matching on literals

What is maybe less well-known, template literal types can be constructed in a similar way as string interpolation.

type Mixed = `${number} ${boolean} ${string}`;

// must follow the format
const mixed: Mixed = "1 true friend";

// Type '"one TRUE friend"' is not assignable to type '`${number} false ${string}` | `${number} true ${string}`'.ts(2322)
const mixed1: Mixed = "one TRUE friend";

In search for symmetry, we will find it's also possible to pattern match on a string literal to recover its components. Here is a type LeadingNumber that extracts the leading number from a string literal:

type LeadingNumber<T extends string> = 
    T extends `${infer N}${infer _}`
    ? N extends `${number}`
        ? N
        : never
    : never;

const leadingNumber: LeadingNumber<'1 true love'> = '1';

// Type '"2"' is not assignable to type '"1"'.ts(2322)
const leadingNumber2: LeadingNumber<'1 true love'> = '2';

Two things to note here,

Recursive parsing

Throw in recursion we can do something quite interesting already. Here is a super strongly-typed Split that splits a string literal by a delimiter that's also a string literal.

// D for delimiter
type Split<D extends string, T extends string> =
    T extends `${infer Head}${D}${infer Tail}`
    ? [Head, ...Split<D, Tail>]
    : [T];

const parts: Split<',', 'you,me,we'> = ['you', 'me', 'we'];

// Type '"yo"' is not assignable to type '"you"'.ts(2322)
const parts2: Split<',', 'you,me,we'> = ['yo', 'me', 'we'];


Now we are ready for the strongly-typed printf.

First thing first, let's agree on the terms: %s, %d, %D are called specifiers; the first string parameter to printf containing the specifiers is called the "format" or "template", to avoid confusion with the TypeScript type "template literal" let's call it "format".

Next we declare the specifiers to be supported. This is done with a map type that maps specifiers to their corresponding types (I know! It's a tongue twister).

type Specifiers = {
    's': string,
    'd': number,
    'b': boolean,
    'D': Date

type Spec = keyof Specifiers;

And using the techniques above, we can locate the specifier in the format string, once found, add the corresponding type to the result array of types. This is done recursively until there is no more specifiers found.

type Values<T extends string> = 
    T extends `${infer _}%${infer K}${infer Rest}`
    ? K extends Spec
        ? [ Specifiers[K], ...Values<Rest> ]
        : Values<`${K}${Rest}`>
    : [];

Note it only works if we help the pattern matching by specifying the % symbol - my guess is otherwise it's hard to decide where to stop inferring K, as it's sandwiched by two other inferences with no constraints or delimiters. With % the specifier is "anchored", therefore easier to locate.

A couple of iterations later we are able to ignore unsupported patterns, and account for edge cases like %%s. Below is the printf in action.

declare function printf<T extends string>(format: T, ...values: Values<T>): string;

const r = printf('this is a %s and it is %d %wyears old, right?%b %D %i %f', 'Hackle', 20, true, new Date());

Naturally we'll also want to restrict the return type to follow the format, other than being a plain string. This is done here.

This post is inspired by the excellent typing for the react router, and the "printf" example from Idris, a truly dependent-typed language.