feat: Incremental building and simultaneous overloading of object values

So we can start with literal objects whose keys are operator names, and
  whose values are arrays of implementations, and merge them, and then
  replace every value with the overloaded function it specifies.

README.md

@@ -8,7 +8,7 @@ Roadmap:
 2. Use the builder pattern to get add working with its implmentations defined incrementally before producing the final overload. [Didn't quite work the way we wnated, but maybe we can do an alternative later.]
 3. Make a version of over.ts, call it util/overload.ts, that takes an array of implementations without redundant type annotation. [DONE]
 4. Improve that version of overload with rtti to select the implementation without the implementations having to throw errors. [DONE]
-3. Use the builder pattern to get a single object with both an add and a negate method, with both defined incrementally, working.
+5. Use the builder pattern to get a single object with both an add and a negate method, with both defined incrementally, working. [DONE]
 4. Incorporate a subtract method that works on numbers and bigint by separate definitions but with dependencies on add and negate.
 5. Incorporate a subtract method that works with one generic implementation that works for both number and bigint with dependencies on add and negate.
 5. Attempt to eliminate redundant specification of implementation signatures.

src/steps/five.ts

@@ -0,0 +1,26 @@
+import 'reflect-metadata'
+import {reflect} from 'typescript-rtti'
+import {merge, overloadValues} from '../util/overload.js'
+
+const numImps = {
+    add: [(x: number, y: number) => x + y],
+    negate: [(x: number) => -x]
+} as const
+
+const strImps = {
+    add: [(x: string, y: string) => x + ', ' + y],
+    negate: [(x: string) => 'NOT ' + x]
+} as const
+
+const merger = merge(numImps)
+const mathImps = merger.with(strImps).imps()
+
+const math = overloadValues(mathImps)
+
+console.log(math.add(1.5, 2.5))
+console.log(math.add('One and a half', 'Two and a half'))
+console.log(math.negate(3.5))
+console.log(math.negate('Three and a half'))
+
+//@ts-expect-error
+console.log(math.add(1.5, 'Two and a half'))

src/util/overload.ts

@@ -10,7 +10,8 @@ export default function overload<T extends readonly [...any[]]>(
     const impTypes = reflect(callSite).parameters[0].elements
     return <any>((...a: any[]) => {
         for (let i = 0; i < imps.length; ++i) {
-            const paramTypes = impTypes[i].type.parameters
+            const paramTypes = imps[i].functionType.parameters
+                || impTypes[i].type.parameters
             let match = true
             const haveArgs = a.length
             let onArg = 0
@@ -40,3 +41,77 @@ export default function overload<T extends readonly [...any[]]>(
             + 'did not match any implementation')
     })
 }
+
+type AVO = Record<string, readonly [...any[]]>
+
+type MergeArrayValues<L extends AVO, R extends AVO> =
+    Pick<L, Exclude<keyof L, keyof R>>
+    & Pick<R, Exclude<keyof R, keyof L>>
+    & { [P in (keyof L & keyof R)]: [...L[P], ...R[P]] }
+
+function mergeImps<T extends AVO, U extends AVO>(
+    impT: T, impU: U): MergeArrayValues<T, U> {
+    const dummy = Object.assign({}, impT, impU)
+    const result = {} as AVO
+    for (const key in dummy) {
+        if (key in impT) {
+            if (key in impU) {
+                const k = key as Extract<keyof T & keyof U, string>
+                result[k] = [...impT[k], ...impU[k]]
+            } else {
+                result[key] = impT[key]
+            }
+        } else {
+            result[key] = impU[key]
+        }
+    }
+    return result as MergeArrayValues<T, U>
+}
+
+class ImpMerger<T extends AVO> {
+    implementations: T
+    constructor(imps: T) {
+        this.implementations = imps
+    }
+    with<U extends AVO>(
+        moreImps: U, callSite?: CallSite): ImpMerger<MergeArrayValues<T,U>> {
+        const Utype = reflect(callSite).parameters[0]
+        // Annotate the implementations with their RTTI while we have it
+        for (const [key, imps] of Object.entries(moreImps)) {
+            if (!(imps[0].functionType)) {
+                const tuple = Utype.members.find(m => m.name === key)
+                for (let i = 0; i < imps.length; ++i) {
+                    imps[i].functionType = tuple.type.elements[i].type
+                }
+            }
+        }
+        return new ImpMerger(mergeImps<T,U>(this.implementations, moreImps))
+    }
+    imps(): T {
+        return this.implementations
+    }
+}
+
+function merge<T extends AVO>(impsObject: T, callSite?: CallSite) {
+    const Ttype = reflect(callSite).parameters[0]
+    // Annotate the implementations with their RTTI while we have it
+    for (const [key, imps] of Object.entries(impsObject)) {
+        if (!(imps[0].functionType)) {
+            const tuple = Ttype.members.find(m => m.name === key)
+            for (let i = 0; i < imps.length; ++i) {
+                imps[i].functionType = tuple.type.elements[i].type
+            }
+        }
+    }
+    return new ImpMerger(impsObject)
+}
+
+type OverloadedValues<T extends AVO> = {
+    [P in keyof T]: UnionToIntersection<T[P][number]>}
+
+function overloadValues<T extends AVO>(impObject: T) : OverloadedValues<T> {
+    return Object.fromEntries(Object.entries(impObject).map(
+        ([k, v]) => [k, overload(v)])) as OverloadedValues<T>
+}
+
+export {merge, mergeImps, overloadValues}