feat(parsers/oneOf): add oneOf parser

src/parsers.ts

@@ -6,6 +6,7 @@ export * from './parsers/float'
 export * from './parsers/integer'
 export * from './parsers/letter'
 export * from './parsers/nothing'
+export * from './parsers/oneOf'
 export * from './parsers/regexp'
 export * from './parsers/rest'
 export * from './parsers/run'

src/parsers/oneOf.ts

@@ -0,0 +1,26 @@
+import { type Parser } from '../state'
+
+export function oneOf(chars: string): Parser<string> {
+  const charset = [...chars]
+
+  return {
+    parse(input, pos) {
+      const nextPos = pos + 1
+      const char = input.substring(pos, nextPos)
+
+      if (charset.includes(char)) {
+        return {
+          isOk: true,
+          pos: nextPos,
+          value: char
+        }
+      }
+
+      return {
+        isOk: false,
+        pos,
+        expected: `one of: ${charset.join(', ')}`
+      }
+    }
+  }
+}