much much faster answer

2015/10/gleam/src/main.gleam

···
+import gleam/bit_array
 import gleam/int
 import gleam/io.{println}
 import gleam/list
···
 import gleam/string.{trim}
 import simplifile.{read}
 
-fn look_and_say(input, output, last_char, last_char_count, i) {
-  let char = string.slice(input, i, 1)
-  case char == last_char {
-    True -> look_and_say(input, output, char, last_char_count + 1, i + 1)
+fn look_and_say(
+  input,
+  output: List(BitArray),
+  last_char: BitArray,
+  last_char_count,
+  i,
+) {
+  let char = bit_array.slice(input, i, 1)
+  case char |> unwrap(<<>>) == last_char {
+    True -> look_and_say(input, output, last_char, last_char_count + 1, i + 1)
     False -> {
-      let output = output <> int.to_string(last_char_count) <> last_char
-      case char != "" {
-        True -> look_and_say(input, output, char, 1, i + 1)
-        False -> output
+      let output =
+        output
+        |> list.prepend(<<int.to_string(last_char_count):utf8>>)
+        |> list.prepend(last_char)
+      case char {
+        Ok(char) -> look_and_say(input, output, char, 1, i + 1)
+        Error(_) -> output
       }
     }
   }
 }
 
 fn look_and_say_helper(input) {
-  let char = string.slice(input, 0, 1)
-  look_and_say(input, "", char, 1, 1)
+  let char = bit_array.slice(<<input:utf8>>, 0, 1) |> unwrap(<<>>)
+  look_and_say(<<input:utf8>>, [], char, 1, 1)
+  |> list.reverse
+  |> bit_array.concat
+  |> bit_array.to_string
+  |> unwrap("")
 }
 
 pub fn main() {
   let input = read(from: "../input.txt") |> unwrap("") |> trim()
 
   let res =
-    list.range(1, 40)
+    list.range(1, 50)
     |> list.fold(input, fn(acc, i) {
-      echo i
+      case i == 41 {
+        True -> println(string.length(acc) |> int.to_string)
+        False -> Nil
+      }
       look_and_say_helper(acc)
     })
   println(string.length(res) |> int.to_string)