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01 — Hello, Mighty

Every Mighty program starts with the same shape: a fn main that the host calls. The standard library exposes a log function for line-based host I/O.

The program

fn main() {
  log("hello, Mighty")
}

What is interesting

  • fn main() declares the entry point. The empty parameter list does not request any capabilities; a host program that does I/O would take capability parameters such as fs: Fs or net: Net (see chapter 10).
  • log(...) is a free function. Mighty does not have implicit print; side-effecting builtins are named and carry effect annotations (see spec §9).
  • The body has no trailing semicolon. Mighty uses block-as-expression semantics: the last expression of a block is its value. A Unit-typed body, like this one, just discards the trailing value.

Try it

mty check examples/01_hello.mty
mty run   examples/01_hello.mty

Expected output:

ok: examples/01_hello.mty
hello, Mighty

You can also see what the compiler builds:

mty dump --hir examples/01_hello.mty

The HIR dump shows a single fn main whose body is a one-statement block calling log with a string literal.

Type errors you might see

The type checker enforces that log takes a single Str:

fn main() { log(42) }   // MT2001 expected `Str`, found `{integer}`
fn main() { log() }     // MT2005 function expects 1 argument(s), got 0

Run mty explain MT2001 for the full Cause/Example/Fix/Spec block.

Next

Continue to 02 — Types.