module examples/runnable/string_chars

-- Demonstrates the chars function for character-by-character processing

-- chars(s) converts a string to a list of single-character strings

-- Unicode-aware: handles emoji and multi-byte characters correctly

import std/string (chars)

import std/list (length, map, filter, foldl)

-- Count vowels in a string

pure func countVowels(s: string) -> int {

let cs = chars(s);

let vowels = filter(func(c: string) -> bool { c == "a" || c == "e" || c == "i" || c == "o" || c == "u" }, cs);

length(vowels)

}

-- Check if string is a palindrome (using character comparison)

pure func isPalindrome(s: string) -> bool {

let cs = chars(s);

let reversed = reverseList(cs);

listsEqual(cs, reversed)

}

pure func reverseList(xs: [string]) -> [string] {

foldl(func(acc: [string], x: string) -> [string] { x :: acc }, [], xs)

}

-- Compare two lists of strings element by element

pure func listsEqual(xs: [string], ys: [string]) -> bool {

match (xs, ys) {

([], []) => true,

(x :: xt, y :: yt) => if x == y then listsEqual(xt, yt) else false,

_ => false

}

}

-- Count total characters (Unicode-aware)

pure func charCount(s: string) -> int {

length(chars(s))

}

-- Extract first character safely

pure func firstChar(s: string) -> string {

match chars(s) {

[] => "",

c :: _ => c

}

}

-- Extract last character safely

pure func lastChar(s: string) -> string {

let cs = chars(s);

match reverseList(cs) {

[] => "",

c :: _ => c

}

}

export func main() -> () ! {IO} {

println("=== chars() Examples ===");

println("");

-- Basic usage

println("Basic usage:");

println(" chars(\"hello\") = ${show(chars("hello"))}");

println(" chars(\"abc\") = ${show(chars("abc"))}");

println("");

-- Unicode handling

println("Unicode handling:");

println(" chars(\"cafe\") = ${show(chars("cafe"))}");

println(" charCount(\"hello\") = ${show(charCount("hello"))}");

println("");

-- Character processing

println("Character processing:");

println(" countVowels(\"hello\") = ${show(countVowels("hello"))}");

println(" countVowels(\"aeiou\") = ${show(countVowels("aeiou"))}");

println("");

-- Palindrome check

println("Palindrome check:");

println(" isPalindrome(\"racecar\") = ${show(isPalindrome("racecar"))}");

println(" isPalindrome(\"hello\") = ${show(isPalindrome("hello"))}");

println("");

-- First/last character

println("First/last character:");

println(" firstChar(\"hello\") = ${show(firstChar("hello"))}");

println(" lastChar(\"hello\") = ${show(lastChar("hello"))}");

println(" firstChar(\"\") = ${show(firstChar(""))}");

()

}