feat(overload): Use typescript-rtti to select implementations (#4)

Note that the changes to module resolution cause steps one and two no longer
  to run because node can't find the imports, although they still compile fine.

Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Reviewed-on: #4

src/steps/four.ts

@@ -0,0 +1,26 @@
+import 'reflect-metadata'
+import overload from '../util/overload.js'
+
+const addImps = [
+    (x: number, y: number) => {
+        if (typeof x === 'number' && typeof y === 'number') return x + y
+        throw new TypeError('Can only add numbers')
+    },
+    (x: string, y: string) => {
+        if (typeof x === 'string' && typeof y === 'string') {
+            return 'Yay' + x + y
+        }
+        throw new TypeError('or strings')
+    },
+    (x: bigint, final?: string) => x.toString() + final
+] as const
+
+const adder = overload(addImps)
+
+console.log(adder(1, 2))
+console.log(adder('a', 'b'))
+console.log(adder(2n))
+
+// And make sure typescript complains on signatures not covered by an imp:
+//@ts-expect-error
+console.log(adder(2n, 4n))

src/steps/one.ts

@@ -1,4 +1,4 @@
-import { useTypes } from 'over.ts/src/index.js';
+import { useTypes } from '../../node_modules/over.ts/src/index.js';
 
 const types = {
    number: (x: unknown): x is number => typeof x === 'number',

src/steps/three.ts

@@ -6,7 +6,10 @@ const adder = overload([
         throw new TypeError('Can only add numbers')
     },
     (x: string, y: string) => 'Yay' + x + y
-])
+] as const)
 
 console.log(adder(1, 2))
 console.log(adder('a', 'b'))
+//@ts-expect-error
+console.log(adder(3n, 4n))
+

src/steps/two.ts

@@ -1,4 +1,4 @@
-import { useTypes } from 'over.ts/src/index.js';
+import { useTypes } from 'over.ts';
 
 const types = {
    number: (x: unknown): x is number => typeof x === 'number',
@@ -69,6 +69,8 @@ addImps.addImp('number, number -> number', (a: number, b: number) => a+b) // ima
 const addAll = addImps.addImp('bigint, bigint -> bigint', (a: bigint, b: bigint) => a+b) // and this is in the `bigint` file
 const add = overload(addAll.finalize() as Omit<typeof addAll, 'finalize'|'addImp'>) // We wrap everything up elsewhere
 
+// This should **NOT** be an error but sadly it is:
+//@ts-expect-error
 console.log('Sum of 5 and 7 is', add(5,7))
 console.log('Sum of 5n and 7n is', add(5n, 7n))
 

src/util/overload.ts

@@ -1,17 +1,42 @@
+import {reflect, CallSite} from 'typescript-rtti'
+
 type UnionToIntersection<U> =
     (U extends any ? (k: U) => void : never) extends ((k: infer I) => void)
     ? I
     : never;
 
 export default function overload<T extends readonly [...any[]]>(
-    imps: T): UnionToIntersection<T[number]> {
+    imps: T, callSite?: CallSite): UnionToIntersection<T[number]> {
+    const impTypes = reflect(callSite).parameters[0].elements
     return <any>((...a: any[]) => {
         for (let i = 0; i < imps.length; ++i) {
-            try {
-                const val = imps[i](...a)
-                return val
-            } catch {
+            const paramTypes = impTypes[i].type.parameters
+            let match = true
+            const haveArgs = a.length
+            let onArg = 0
+            for (const param of paramTypes) {
+                if (param.isRest) {
+                    // All the rest of the arguments must be of param's type
+                    match = a.slice(onArg).every(
+                        arg => param.type.matchesValue(arg))
+                    break
+                }
+                if (onArg === haveArgs) {
+                    // We've used all of the arguments, so better be optional
+                    match = param.isOptional
+                    break
+                }
+                // This argument must match this param's type and both are used
+                match = param.type.matchesValue(a[onArg])
+                onArg += 1
+                if (!match) break
             }
+            // Make sure we used all the arguments
+            match &&= (onArg == haveArgs)
+            if (match) return imps[i](...a)
         }
+        throw new TypeError(
+            `Actual arguments ${a} of type ${a.map(arg => typeof arg)} `
+            + 'did not match any implementation')
     })
 }