Typescript: strongly typed, lazy evaluated, interpolated strings

template_strings.ts

let id = (x: number) => x;
let param = (x: string) => x;

type ParamType<T> = T extends (arg: infer R) => any ? R : never;

/**
 * Templates a string by substituting placeholders for tokens later in execution.
 * It is designed to work as the tag function for tagged interpolated strings.
 * @returns A reusable template function that is statically checked for arity and 
 * parameter type compatibility with the placeholders from the interpolated string.
 * @param strings The strings around the tokens to template
 * @param placeholders Placeholders that are substituted for token when
 *  templating is done. Note these are transform functions.
 */
function makeTemplate<T extends ((any: any) => any)[]>(
    strings: TemplateStringsArray,
    ...placeholders: T) {

    // https://github.com/microsoft/TypeScript/issues/12754 should improve this even more
    return function template(...tokens: { [K in keyof T]: ParamType<T[K]> }) {
        // interleave the strings with the parameters
        let result = Array(strings.length + tokens.length);

        for (let i = 0; i < tokens.length; i++) {
            result[i * 2] = strings[i];
            result[i * 2 + 1] = placeholders[i](tokens[i]);
        }

        result[result.length - 1] = strings[strings.length - 1];

        return result.join('');
    }
}

// let url: (tokens_0: number, tokens_1: string, tokens_2: number, tokens_3: string) => string
let url = makeTemplate`${id}/projects/${param}/sites/${id}/new/${param}`

class AudioRecording { id; }
let audioRecordingId = (p: AudioRecording) => p.id;
// let url2: (tokens_0: AudioRecording, tokens_1: number) => string
let url2 = makeTemplate`/audio_recordings/${audioRecordingId}/audio_events/${id}/new/`


console.log("final:");
console.log(url);                                // [Function: template]
console.log(url(3, "hello", 2, "world"));        // 3/projects/hello/sites/2/new/world
//console.log(url(3, "hello", 2, "world", 6));   // error: Expected 4 arguments, but got 5.ts(2554)
//console.log(url());                            // error: Expected 4 arguments, but got 0.ts(2554)
//console.log(url(3, 4, 2, "world"));            // error: Argument of type '4' is not assignable to parameter of type 'string'.ts(2345)

//console.log(url2(3, "hello"));                 // error: Argument of type '3' is not assignable to parameter of type 'AudioRecording'.ts(2345)
let ar = { id: 12345} as AudioRecording
console.log(url2(ar, 6789));                     // /audio_recordings/12345/audio_events/6789/new/