Hello, world

A Yon program is a list of top-level declarations; execution starts at main, which returns a number:

fun main(): number {
  return 0
}

Compile and run it with yonc, the end-to-end compiler (OCaml frontend → the Topos MLIR dialect → LLVM → a native executable):

$ yonc hello.yon -o hello
$ ./hello

The process exit code is main's return value, truncated mod 256, and Yon programs commonly use it as their observable result:

fun main(): number {
  return 300   // the process exit code is main's value mod 256 -> 44
}

Printing takes one call:

fun main(): number {
  be _p holds String.print("ciao, mondo")
  return 0
}

Two things are already visible here. First, be x holds e is the binding form of Yon, immutable, used everywhere (there is no let). Second, the string literal is a real value: since the 1.0 string fusion, "ciao, mondo" is a section of the builtin String place, interned on the content-addressed heap.

String.print is a direct builtin. Yon also has an effectful printing path (Output.print under the visits Output effect), we will meet it in the chapter on functions and effects.

You can stop the compiler at any stage and look at what it produced:

$ yonc --emit=mlir hello.yon      # the Topos-dialect MLIR from the frontend
$ yonc --emit=standard hello.yon  # after the Topos -> standard lowering
$ yonc --emit=ll hello.yon        # the LLVM IR