import { Kind, Type } from '..';
type _$letter = 'a' | 'b' | 'c' | 'd' | 'e' | 'f' | 'g' | 'h' | 'i' | 'j' | 'k' | 'l' | 'm' | 'n' | 'o' | 'p' | 'q' | 'r' | 's' | 't' | 'u' | 'v' | 'w' | 'x' | 'y' | 'z' | 'A' | 'B' | 'C' | 'D' | 'E' | 'F' | 'G' | 'H' | 'I' | 'J' | 'K' | 'L' | 'M' | 'N' | 'O' | 'P' | 'Q' | 'R' | 'S' | 'T' | 'U' | 'V' | 'W' | 'X' | 'Y' | 'Z';
/**
 * `_$isLetter` is a type-level function that checks if
 *
 * @template {string} S - The string to check.
 *
 * @example
 * ```
 * type T0 = String._$isLetter<'f'> // true
 * type T1 = String._$isLetter<'9'> // false
 * ```
 */
export type _$isLetter<S extends string> = S extends _$letter ? true : false;
/**
 * `IsLetter` is a type-level function that checks if a string is a letter.
 *
 * @template {string} S - The string to check.
 *
 * @example
 * ```ts
 * import { $, String } from "hkt-toolbelt";
 *
 * type Result = $<String.IsLetter, 'f'> // true
 * ```
 */
export interface IsLetter extends Kind.Kind {
    f(x: Type._$cast<this[Kind._], string>): _$isLetter<typeof x>;
}
/**
 * Given a string, return whether or not it is a letter.
 *
 * @param {string} x - The string to check.
 *
 * @example
 * ```ts
 * import { String } from "hkt-toolbelt";
 *
 * const result = String.isLetter('f')
 * //    ^? true
 * ```
 */
export declare const isLetter: Kind._$reify<IsLetter>;
export {};