Add types for function-partial (#437)

packages/function-partial/index.d.ts

@@ -0,0 +1,19 @@
+// https://github.com/ronami/meta-typing/blob/master/src/tail/index.d.ts
+type Tail<T extends Array<any>> =
+  ((...t: T) => void) extends (h: any, ...rest: infer R) => void ? R : never;
+
+/**
+ * Return a list of Args such that the corresponding value is undefined. 
+ * @example PartialArgs<[number, string, boolean], [number, undefined, undefined]> equals to [string, boolean]
+ * @example PartialArgs<[number, string, ...boolean[]], [undefined, string]> 
+ *          equals to [number, ...boolean[]]
+ */
+type PartialArgs<Args extends any[], Values extends any[], Output extends any[] = []>
+  = [] extends Values
+  ? [...Output, ...Args]
+  : undefined extends Values[0]
+  ? PartialArgs<Tail<Args>, Tail<Values>, [...Output, Args[0]]>
+  : PartialArgs<Tail<Args>, Tail<Values>, Output>
+
+export function partial<Values extends any[], F extends (...args: any) => any>(func: F, ...args: Values)
+  : (...args: PartialArgs<Parameters<F>, Values>) => ReturnType<F>

packages/function-partial/index.tests.ts

@@ -0,0 +1,106 @@
+import { partial } from './index';
+
+// Case 1) one aurgument.
+const sqrt = partial(Math.sqrt)
+sqrt(16)
+
+// @ts-expect-error
+sqrt()
+// @ts-expect-error
+sqrt(2, 2)
+// @ts-expect-error
+sqrt(true)
+
+const sqrtOf16 = partial(Math.sqrt, 16)
+sqrtOf16()
+
+// @ts-expect-error
+sqrtOf16(16)
+
+// Case 2) Two arguments.
+const repeat = (str: string, n: number) => {
+  return str.repeat(n)
+}
+
+const anotherRepeat = partial(repeat)
+anotherRepeat("a", 2)
+
+// @ts-expect-error
+anotherRepeat()
+// @ts-expect-error
+anotherRepeat("a")
+// @ts-expect-error
+anotherRepeat(2)
+// @ts-expect-error
+anotherRepeat(2, "a")
+
+const repeatHello = partial(repeat, "hello, ")
+repeatHello(10)
+
+// @ts-expect-error
+repeatHello()
+// @ts-expect-error
+repeatHello([])
+
+const repeat3Times = partial(repeat, undefined, 3)
+repeat3Times('hello, ')
+
+// @ts-expect-error
+repeat3Times()
+// @ts-expect-error
+repeat3Times({})
+
+const repeatHello3Times = partial(repeat, "hello, ", 3)
+repeatHello3Times()
+
+// @ts-expect-error
+repeatHello3Times(1, 2, 3)
+
+// Case 3) three arguments.
+const getFormattedPrice = (amount: number, unit: string, includeTax: boolean) => {
+  const tax = includeTax ? 1.1 : 1
+  return `${unit}${amount * tax}`
+}
+
+const anotherGetFormattedPrice = partial(getFormattedPrice)
+anotherGetFormattedPrice(100, "$", true)
+
+// @ts-expect-error
+anotherGetFormattedPrice({})
+
+const getFormattedPriceWithTax = partial(getFormattedPrice, undefined, undefined, true)
+getFormattedPriceWithTax(100, "€")
+
+const getFormattedPriceInUS = partial(getFormattedPrice, undefined, "$", undefined)
+getFormattedPriceInUS(100, false)
+
+const getFormattedCookiePrice = partial(getFormattedPrice, 100, undefined, undefined)
+getFormattedCookiePrice("$", false)
+
+const getFormattedCookiePriceInUS = partial(getFormattedPrice, 100, "$", undefined)
+getFormattedCookiePriceInUS(true)
+
+const getFormattedCookiePriceWithoutTax = partial(getFormattedPrice, 100, undefined, false)
+getFormattedCookiePriceWithoutTax("¥")
+
+const getFormattedPriceWithTaxInUS = partial(getFormattedPrice, undefined, "$", true)
+getFormattedPriceWithTaxInUS(10000)
+
+const getFormattedCookiePriceWithTaxInUS = partial(getFormattedPrice, 100, "$", true)
+getFormattedCookiePriceWithTaxInUS()
+
+// Case 4) arbitrarily many arguments.
+const consoleLog = partial(console.log)
+consoleLog("hello", ", ", "world")
+
+const logHello = partial(console.log, "Hello")
+logHello()
+
+const logHelloWorld = partial(console.log, "Hello", ", ", "world")
+logHelloWorld()
+
+const logGreetWorld = partial(console.log, undefined, ", ", "world")
+logGreetWorld("Good Afternoon")
+
+const logGreetToSomeone = partial(console.log, undefined, ", ")
+logGreetToSomeone("Good Afternoon", "World", "!")

packages/function-partial/package.json

@@ -10,6 +10,7 @@
       "default": "./index.mjs"
     }
   },
+  "types": "index.d.ts",
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1",
     "build": "rollup -c"
@@ -26,4 +27,4 @@
   "bugs": {
     "url": "https://github.com/angus-c/just/issues"
   }
-}
+}