The SLC Programming Language

SLC is a programming language. A program is a command: it sends a value to a continuation, and that meeting is a step of the program. Its core follows the classical λ̄μμ̃ calculus (lambda-bar-mu-mu-tilde). Source files end in .sl. The command-line tool is slc.

There is one grammar. A type denotes a value or a continuation, and the arrow says which way a function faces, so the same program can be written value-first or continuation-first. Values and Continuations are those two halves. Data gives them the same shapes: data and form, enum and menu.

This book is the tutorial and the reference for the language implemented in the SLC repository. The specification is DESIGN.md, with the parts under docs/design/. Where this book and the specification disagree, the specification and the compiler decide.

The programs printed in the tutorial live in book/examples/. book/check.sh runs them and compares the output.

Two ways through one pipeline

A function written with -> is given a value and returns a value.

func area(s: Shape) -> i64 {
    of s {
        Circle(r) => <(3, r) | mul | x => (x, r) | mul,
        Rect(w, h) => <(w, h) | mul,
    }
}

of s takes the shape apart. With 3 standing in for π, the circle of radius 5 has area 75.

A function written with <- is given a continuation of the result and returns a continuation of the input.

func area_of(out: i64) <- Shape {
    mu Shape {
        Circle(r) => <(3, r) | mul | x => (x, r) | mul | out>,
        Rect(w, h) => <(w, h) | mul | out>,
    }
}

out: i64 is the continuation of the area. mu Shape builds the continuation of the shape, and each arm sends the area to out.

< opens a chain with a value, and | carries that value from left to right. A chain reads each stage in the orientation that stage was written with, so both functions sit in the same pipeline. Both arrows runs this program. The whole dual example in the repository is examples/duality/two_styles.sl.