Your First Program¶

Running a file¶

Create a file called hello.scm:

(import (scheme base) (scheme write))

(display "Hello, world!")
(newline)

Run it:

kaappi hello.scm

Output:

Hello, world!

The REPL¶

Launch the REPL with no arguments:

kaappi

Kaappi Scheme v0.1.0
Type (exit) to quit.

kaappi>

The REPL provides:

Line editing -- arrow keys, Ctrl-A (start of line), Ctrl-E (end of line), backspace, delete
Command history -- up/down arrows, persisted across sessions in .kaappi_history
Tab completion -- completes all built-in and user-defined symbols
Multi-line input -- open parentheses are tracked; the prompt changes to ... until all parens are balanced

kaappi> (define (square x)
  ...     (* x x))
kaappi> (square 7)
49
kaappi> (map square '(1 2 3 4 5))
(1 4 9 16 25)

Type (exit) or press Ctrl-D to quit.

No install yet?

You can try all these examples in the browser playground — same interpreter, no installation required.

A Multi-File Project¶

Real programs split code into libraries. Here's a small project that computes statistics on a list of numbers.

Create the project structure:

stats-demo/
  mylib/
    stats.sld
  main.scm

Write the library in mylib/stats.sld:

(define-library (mylib stats)
  (export mean median)
  (import (scheme base)
          (srfi 1)    ;; for fold
          (srfi 132)) ;; for list-sort
  (begin
    (define (mean lst)
      (/ (fold + 0 lst) (length lst)))

    (define (median lst)
      (let* ((sorted (list-sort < lst))
             (n (length sorted))
             (mid (quotient n 2)))
        (if (odd? n)
            (list-ref sorted mid)
            (/ (+ (list-ref sorted (- mid 1))
                  (list-ref sorted mid))
               2))))))

Write the main program in main.scm:

(import (scheme base)
        (scheme write)
        (mylib stats))

(define data '(7 2 9 4 1 8 3 6 5))

(display "Data:   ") (display data)   (newline)
(display "Mean:   ") (display (inexact (mean data))) (newline)
(display "Median: ") (display (median data)) (newline)

Run it from inside stats-demo/:

cd stats-demo
kaappi main.scm

Output:

Data:   (7 2 9 4 1 8 3 6 5)
Mean:   5.0
Median: 5

The key points:

Library name (mylib stats) maps to file path mylib/stats.sld
export controls what the library exposes
import in both the library and main program pulls in dependencies
Kaappi finds mylib/stats.sld automatically from the current directory

For more on libraries, import modifiers, and search paths, see Working with Libraries.

Next: Scheme Tutorial