A first program

// The smallest SLC program.

proc main | (exit: i32) / {IO} {
    <"Hello, SLC!" | println;
    <0 | exit>
}

Run it from the repository:

slc run book/examples/hello.sl

It prints

Hello, SLC!

and exits with status 0.

The entry point

proc main is the entry point. It takes no values and one continuation, exit. The runtime supplies that continuation. The integer sent to exit is the process status.

{IO} is the effect of printing. The runtime handles it. A main that only sends a status, and prints nothing, can leave the row off. A func main, a main with value parameters, and a main inside a section are refused. The diagnostic names the shape proc main | (exit: -i32) / {IO}.

Every terminating path of main sends a status to exit. Falling off the end is a type error. There is no separate exit value sitting beside main: ending the program is the continuation main was given, and a helper receives it only when main passes it on.

A chain

< opens a chain with a value. | carries that value to the next stage. <"Hello, SLC!" | println sends the string to println. The semicolon ends the statement, and the block continues. The chain has no closing >, so it is an application: println returns the unit value (,) after writing the line.

<0 | exit> is a cut. The > delivers 0 to the continuation exit and does not come back. A cut is how a command ends.

A chain says what it is at both ends. < makes it start from a value, > makes it deliver to a continuation, and the two together are a cut. "Hello, SLC!" | println is refused, because a string is a value and a chain without < starts with a function. <0 alone is refused, because the value has nowhere to go.

println renders its argument through Display and performs IO. print is the same operation without a trailing newline.