Add dropPrefix and dropSuffix to String

CHANGELOG.md

@@ -9,7 +9,7 @@ This project adheres to semantic versioning.
 - Add an `Eq` instance to `URLPath`.
 - Add `singleton`, `appendAt`, `upsertAt`, `deleteAt`, `lookup`, `lookupFirst`, `toLeadingString`, `keys`, `values`, `size`, `concatBy`, `fromMap` and `toMap`, and `Eq`, `Semigroup` and `Monoid` instances to `URLSearchParams`.
 - Add `integerFromString` to `Number`.
-- Add `fromBool` to `String`.
+- Add `dropPrefix`, `dropSuffix`, and `fromBool` to `String`.
 - Fix `fromString` in `Number` not parsing floats.
 - Deprecate `getParam`, `getAllForParam`, and `setParam` in `URLSearchParams`.
 

docs/modules/String.ts.md

@@ -18,8 +18,10 @@ Added in v0.1.0
   - [append](#append)
   - [dropLeft](#dropleft)
   - [dropLeftWhile](#dropleftwhile)
+  - [dropPrefix](#dropprefix)
   - [dropRight](#dropright)
   - [dropRightWhile](#droprightwhile)
+  - [dropSuffix](#dropsuffix)
   - [fromBool](#frombool)
   - [fromNumber](#fromnumber)
   - [head](#head)
@@ -142,6 +144,34 @@ assert.strictEqual(dropFilename('File.hs'), '.hs')
 
 Added in v0.6.0
 
+## dropPrefix
+
+Drop a prefix if present, else return the input string unmodified.
+
+**Signature**
+
+```ts
+export declare const dropPrefix: (prefix: string) => Endomorphism<string>
+```
+
+```hs
+dropPrefix :: string -> Endomorphism string
+```
+
+**Example**
+
+```ts
+import { dropPrefix } from 'fp-ts-std/String'
+
+const f = dropPrefix('foo')
+
+assert.strictEqual(f('foobar'), 'bar')
+assert.strictEqual(f('barfoo'), 'barfoo')
+assert.strictEqual(f('foofoo'), 'foo')
+```
+
+Added in v0.18.0
+
 ## dropRight
 
 Drop a number of characters from the end of a string, returning a new
@@ -204,6 +234,34 @@ assert.deepStrictEqual(dropRightVowels('hellooo'), 'hell')
 
 Added in v0.7.0
 
+## dropSuffix
+
+Drop a suffix if present, else return the input string unmodified.
+
+**Signature**
+
+```ts
+export declare const dropSuffix: (suffix: string) => Endomorphism<string>
+```
+
+```hs
+dropSuffix :: string -> Endomorphism string
+```
+
+**Example**
+
+```ts
+import { dropSuffix } from 'fp-ts-std/String'
+
+const f = dropSuffix('bar')
+
+assert.strictEqual(f('foobar'), 'foo')
+assert.strictEqual(f('barfoo'), 'barfoo')
+assert.strictEqual(f('barbar'), 'bar')
+```
+
+Added in v0.18.0
+
 ## fromBool
 
 Convert a boolean to a string.

src/String.ts

@@ -683,3 +683,39 @@ export const words = S.split(/\s/)
  * @since 0.14.0
  */
 export const unwords = join(" ")
+
+/**
+ * Drop a prefix if present, else return the input string unmodified.
+ *
+ * @example
+ * import { dropPrefix } from 'fp-ts-std/String'
+ *
+ * const f = dropPrefix('foo')
+ *
+ * assert.strictEqual(f('foobar'), 'bar')
+ * assert.strictEqual(f('barfoo'), 'barfoo')
+ * assert.strictEqual(f('foofoo'), 'foo')
+ *
+ * @category 3 Functions
+ * @since 0.18.0
+ */
+export const dropPrefix = (prefix: string): Endomorphism<string> =>
+  when(S.startsWith(prefix))(dropLeft(S.size(prefix)))
+
+/**
+ * Drop a suffix if present, else return the input string unmodified.
+ *
+ * @example
+ * import { dropSuffix } from 'fp-ts-std/String'
+ *
+ * const f = dropSuffix('bar')
+ *
+ * assert.strictEqual(f('foobar'), 'foo')
+ * assert.strictEqual(f('barfoo'), 'barfoo')
+ * assert.strictEqual(f('barbar'), 'bar')
+ *
+ * @category 3 Functions
+ * @since 0.18.0
+ */
+export const dropSuffix = (suffix: string): Endomorphism<string> =>
+  when(S.endsWith(suffix))(dropRight(S.size(suffix)))

test/String.ts

@@ -37,6 +37,8 @@ import {
   isSpace,
   words,
   unwords,
+  dropPrefix,
+  dropSuffix,
 } from "../src/String"
 import * as O from "fp-ts/Option"
 import * as NEA from "fp-ts/NonEmptyArray"
@@ -796,4 +798,44 @@ describe("String", () => {
       expect(f(["a", "b", "c"])).toBe("a b c")
     })
   })
+
+  describe("dropPrefix", () => {
+    const f = dropPrefix
+
+    it("removes prefix if present", () => {
+      expect(f("foo")("foobar")).toBe("bar")
+
+      fc.assert(
+        fc.property(fc.string(), fc.string(), (x, y) => f(y)(y + x) === x),
+      )
+    })
+
+    it("does not modify string without prefix", () => {
+      expect(f("bar")("foobar")).toBe("foobar")
+    })
+
+    it("does not modify string given empty prefix", () => {
+      fc.assert(fc.property(fc.string(), x => f("")(x) === x))
+    })
+  })
+
+  describe("dropSuffix", () => {
+    const f = dropSuffix
+
+    it("removes suffix if present", () => {
+      expect(f("bar")("foobar")).toBe("foo")
+
+      fc.assert(
+        fc.property(fc.string(), fc.string(), (x, y) => f(y)(x + y) === x),
+      )
+    })
+
+    it("does not modify string without suffix", () => {
+      expect(f("foo")("foobar")).toBe("foobar")
+    })
+
+    it("does not modify string given empty suffix", () => {
+      fc.assert(fc.property(fc.string(), x => f("")(x) === x))
+    })
+  })
 })