REPL Guide

Launch the interactive REPL with no arguments:

kaappi

Kaappi Scheme v0.2.1
Type (exit) to quit.

kaappi> (+ 1 2)
3
kaappi> _
3

Line Editing

The REPL uses linenoise for line editing with full terminal support.

Key	Action
Left / Right	Move cursor
Up / Down	Navigate history
Ctrl+A	Move to start of line
Ctrl+E	Move to end of line
Ctrl+K	Delete from cursor to end
Ctrl+U	Delete entire line
Ctrl+W	Delete previous word
Ctrl+L	Clear screen
Ctrl+D	Delete char at cursor (or exit if line empty)
Ctrl+T	Transpose characters
Tab	Auto-complete symbol names
Ctrl+R	Reverse history search
Ctrl+C	Cancel current line

Tab Completion

Press Tab to complete symbol names from the global environment:

kaappi> string-<TAB>
string-append  string-copy  string-length  string-ref  ...

Completion matches all bound symbols — built-in procedures, your own definitions, and imported library exports.

History

History is saved automatically to .kaappi_history in the current directory. Up to 1000 entries are preserved across sessions.

• Up / Down arrows — navigate through previous entries
• Ctrl+P / Ctrl+N — same as Up / Down

Reverse History Search (Ctrl+R)

Press Ctrl+R to incrementally search through history, just like bash:

kaappi> <Ctrl+R>
(reverse-i-search)'': 

Type a search query — matching history entries appear as you type:

(reverse-i-search)'def': (define (fib n) (if (< n 2) n (+ (fib (- n 1)) (fib (- n 2)))))

Key	Action
Type characters	Refine search
Ctrl+R	Jump to next (older) match
Enter	Accept the match for editing
Escape	Cancel and restore original line
Backspace	Remove last search character

Multi-line Input

Expressions with unmatched parentheses automatically continue on the next line:

kaappi> (define (square x)
  ...    (* x x))
kaappi> (square 7)
49

The prompt changes to ... while input is incomplete. Press Ctrl+C to cancel a multi-line entry.

The _ Variable

The special variable _ always holds the result of the last evaluation:

kaappi> (* 6 7)
42
kaappi> (+ _ 8)
50
kaappi> (string-append "answer: " (number->string _))
"answer: 50"

This is useful for exploratory programming — evaluate something, then use the result without re-typing or binding it.

REPL Commands

All commands start with a comma (,). They are not Scheme expressions.

Inspection

,type <expr> — Show result type

kaappi> ,type (+ 1 2)
; integer
kaappi> ,type "hello"
; string
kaappi> ,type (list 1 2 3)
; pair
kaappi> ,type car
; procedure
kaappi> ,type #t
; boolean

,describe <symbol> — Show binding details

Shows the type, arity, and source location of a procedure:

kaappi> ,describe car
  car
    type: procedure
    arity: 1
kaappi> ,describe map
  map
    type: procedure
    arity: 2+
kaappi> ,describe +
  +
    type: procedure
    arity: 0+

For user-defined procedures, it also shows the source file and line:

kaappi> (define (greet name) (string-append "Hello, " name))
kaappi> ,describe greet
  greet
    type: procedure
    arity: 1, locals: 0
    source: <repl>:1

,apropos <string> — Search bindings

Searches all global bindings for names containing the given substring:

kaappi> ,apropos vector
  vector-append
  vector-copy
  vector-fill!
  vector-for-each
  vector-length
  vector-map
  vector-ref
  vector-set!
  vector?
  list->vector
  vector->list
  make-vector
  ...
; 18 matches

,env [prefix] — List bindings

Lists all global bindings, optionally filtered by prefix:

kaappi> ,env string-
  string-append
  string-copy
  string-length
  ...
; 24 bindings

Timing and Profiling

,time <expr> — Measure execution time

kaappi> ,time (fib 30)
832040
; 0.173 seconds

,profile <expr> — Detailed profiling

Shows per-function timing, call counts, and memory allocations:

kaappi> ,profile (fib 25)
75025
; Profile Report
; ...

Macros

,expand <expr> — Show macro expansion

kaappi> ,expand (when #t (display "yes"))
(if #t (begin (display "yes")))

Debugging

,break <name> — Set breakpoint

kaappi> ,break fib
Breakpoint set on fib

,breakpoints — List breakpoints

kaappi> ,breakpoints
  [0] fib

,step <expr> — Single-step evaluation

kaappi> ,step (+ 1 2)

,delete all — Clear all breakpoints

kaappi> ,delete all
All breakpoints deleted

Runtime

,gc — Show GC statistics

kaappi> ,gc
GC Statistics:
  Collections:       42
  Live objects:      1523 (peak: 3201)
  Heap size:         48736 bytes (peak: 102400)
  ...

,help — Show all commands

kaappi> ,help
Commands:
  ,time <expr>      Measure execution time
  ,type <expr>      Show result type
  ,describe <sym>   Show procedure arity and type
  ,apropos <str>    Search bindings by substring
  ,env [prefix]     List global bindings by prefix
  ,profile <expr>   Profile timing, calls, and allocations
  ,expand <expr>    Show macro expansion
  ,gc               Show GC statistics
  ,break <name>     Set breakpoint on function
  ,breakpoints      List active breakpoints
  ,delete all       Clear all breakpoints
  ,step <expr>      Evaluate with single-stepping
  ,help             This message

The variable _ holds the last result.

Exiting

kaappi> (exit)

Or press Ctrl+D on an empty line.

---

Next: Tips