🐛 Improve type inference

common/init.ts

@@ -0,0 +1,87 @@
+// Copyright 2021-present the Fonction authors. All rights reserved. MIT license.
+import { slice } from '../deps.ts'
+
+/**
+ * @example
+ * ```ts
+ * InitString<string> // string
+ * InitString<''> // ''
+ * InitString<'a'> // ''
+ * InitString<'ab'> // 'a'
+ * InitString<'abcd'> // 'abc'
+ * ```
+ * @internal
+ */
+type InitString<T extends string> = T extends `${infer F}${infer R}`
+  ? R extends ''
+    ? ''
+    : `${F}${InitString<R>}`
+  : T extends ''
+  ? ''
+  : string
+
+/**
+ * Infer the init types.
+ * @typeParam T - `string` or any `array`
+ *
+ * @example
+ * ```ts
+ * // String
+ * Init<string> // string
+ * Init<''> // ''
+ * Init<'hello'> // 'hell'
+ * ```
+ *
+ * @example
+ * ```ts
+ * // Array
+ * Init<[] | never[] | readonly [] | readonly never[]> // []
+ * Init<['hello', 'world']> // 'hello'
+ * ```
+ *
+ * @category `Array` `String`
+ *
+ * @see Related to {@link First}
+ *
+ * @public
+ */
+type Init<T extends string | readonly unknown[]> = T extends string
+  ? InitString<T>
+  : T extends readonly never[] | []
+  ? []
+  : T extends readonly [...infer I, unknown]
+  ? I
+  : T
+
+/**
+ * Returns all but the init element of the given list or string.
+ * @param val - string or any array object
+ * @returns The result of `val.slice(0, -1)`
+ *
+ * @example
+ * ```ts
+ * // String
+ * init('hello') // 'hell'
+ * init('h') // ''
+ * init('') // ''
+ * ```
+ *
+ * @example
+ * ```ts
+ * init([1, 2, 3]) // [1, 2]
+ * init(['hello', 'world']) // ['hello']
+ * init(['hello']) // []
+ * init([]) // []
+ * ```
+ *
+ * @category `Array` `String`
+ *
+ * @see Related to {@link tail}
+ *
+ * @public
+ */
+const init = <T extends string | readonly unknown[]>(val: T): Init<T> =>
+  slice(0, -1, val) as Init<T>
+
+export { init }
+export type { Init }

test/init.test.ts → common/init_test.ts

@@ -1,6 +1,6 @@
 // Copyright 2021-present the Fonction authors. All rights reserved. MIT license.
-import { assertEquals } from '../dev_deps.ts'
-import { init } from '../src/init.ts'
+import { assertEquals, assertEqualsType } from '../dev_deps.ts'
+import { Init, init } from './init.ts'
 
 Deno.test('init', () => {
   const tableString: [string, string][] = [
@@ -37,4 +37,14 @@ Deno.test('init', () => {
   tableArray.forEach(([val, expected]) => {
     assertEquals(init(val), expected, `init(${val}) -> ${expected}`)
   })
+
+  assertEqualsType<[], Init<[]>>()
+  assertEqualsType<readonly [], Init<readonly []>>()
+  assertEqualsType<[], Init<[1]>>()
+  assertEqualsType<[1], Init<[1, 2]>>()
+  assertEqualsType<[1, 2], Init<[1, 2, 3]>>()
+  assertEqualsType<'', Init<''>>()
+  assertEqualsType<'', Init<'a'>>()
+  assertEqualsType<'a', Init<'ab'>>()
+  assertEqualsType<'abcde', Init<'abcdef'>>()
 })

common/mod.ts

@@ -1,5 +1,7 @@
 // Copyright 2021-present the Fonction authors. All rights reserved. MIT license.
 export type { Head } from './head.ts'
 export { head } from './head.ts'
+export type { Init } from './init.ts'
+export { init } from './init.ts'
 export type { Last } from './last.ts'
 export { last } from './last.ts'

deps.ts

@@ -1,5 +1,6 @@
 // Copyright 2021-present the Fonction authors. All rights reserved. MIT license.
-export { length } from 'https://deno.land/x/[email protected]/mod.ts'
+export { length } from 'https://deno.land/x/[email protected]/mod.ts'
+export { slice } from 'https://deno.land/x/[email protected]/uncurry/common/slice.ts'
 export { curry } from 'https://deno.land/x/[email protected]/mod.ts'
 export { equal } from 'https://deno.land/x/[email protected]/mod.ts'
 export {

dev_deps.ts

@@ -1,8 +1,12 @@
 // Copyright 2021-present the Fonction authors. All rights reserved. MIT license.
+/* eslint-disable @typescript-eslint/no-empty-function */
+/* eslint-disable @typescript-eslint/no-unused-vars */
+
 export {
   assert,
   assertEquals
 } from 'https://deno.land/[email protected]/testing/asserts.ts'
+export const assertEqualsType = <T, U extends T = T>(_actual?: U): void => {}
 export { length } from 'https://deno.land/x/[email protected]/mod.ts'
 export {
   isNumber,

mod.ts

@@ -22,7 +22,6 @@ export { identity } from './src/identity.ts'
 export { ifElse } from './src/ifElse.ts'
 export { ifElseFn } from './src/ifElseFn.ts'
 export { inc } from './src/inc.ts'
-export { init } from './src/init.ts'
 export { K } from './src/K.ts'
 export { lt } from './src/lt.ts'
 export { lte } from './src/lte.ts'