main.gleam

import writer
import string_writer

pub fn main() {
  string_writer.create()
  |> writer.write("Text1")
  |> writer.write("Text2")
  |> string_writer.print()
}

string_writer.gleam

import writer.{Writer}
import gleam/string_builder.{StringBuilder}
import gleam/io

pub fn create() -> Writer(StringBuilder) {
  Writer(
    target: string_builder.from_strings([]),
    write: fn(writer, str) {
      let assert Writer(builder, write) = writer
      let next_builder = string_builder.append(builder, str)
      Writer(next_builder, write)
    },
  )
}

pub fn print(writer: Writer(StringBuilder)) {
  let assert Writer(builder, _) = writer
  io.print(string_builder.to_string(builder))
}

writer.gleam

pub type Writer(a) {
  Writer(target: a, write: fn(Writer(a), String) -> Writer(a))
}

pub fn write(writer: Writer(a), str) {
  let assert Writer(_, write) = writer
  write(writer, str)
}

Idea to simulate Iterfaces in gleam ?