fun> Fun - Fun Handbook (Second Edition)

This is a refreshed, de-duplicated, and fully up-to-date handbook for the Fun programming language and its virtual machine (VM). It keeps the same section layout as the original handbook while consolidating repeated content and documenting all currently available features, including the latest SQLite support.

Overview

Fun is a small, strict, and simple programming language executed by a stack-based virtual machine. Most of the ecosystem is written in Fun itself; only a minimal core is implemented in C. The design focuses on simplicity, consistency, and joy in coding.

Introduction

• Dynamic and optionally statically typed
• Type safety
• Written in C (C99) and Fun
• Minimal C core; most core functions and libraries implemented in Fun
• Internal libraries use snake_case for functions even when written in Fun; class names are CamelCase

Installation

Requirements

• A C compiler, a libc, and Git

FreeBSD:

• CMake
• Clang

Linux:

• CMake
• GCC (Clang should also work)

Windows:

• Cygwin (not covered in detail here)
• CMake
• GCC via Cygwin

Build Fun

Linux/UNIX and Cygwin are covered here.

Clone repository:

git clone https://git.xw3.org/fun/fun.git
cd fun

Configure and build (examples shown with several optional features enabled):

cmake -S . -B build \
  -DFUN_DEBUG=OFF \
  -DFUN_WITH_REPL=ON \
  -DFUN_WITH_JSON=ON \
  -DFUN_WITH_PCRE2=ON \
  -DFUN_WITH_CURL=ON \
  -DFUN_WITH_PCSC=OFF \
  -DFUN_WITH_SQLITE=OFF
cmake --build build --target fun

Run the demo (without installing):

FUN_LIB_DIR="$(pwd)/lib" ./build/fun ./demo.fun

Tracing execution:

FUN_LIB_DIR="$(pwd)/lib" ./build/fun --trace ./demo.fun

Drop into the REPL when an error occurs:

FUN_LIB_DIR="$(pwd)/lib" ./build/fun --repl-on-error --trace ./demo.fun

Start the REPL directly (build with -DFUN_WITH_REPL=ON):

FUN_LIB_DIR="$(pwd)/lib" ./build/fun

CMake options

Pass all options as -DNAME=VALUE. The most relevant toggles are:

• FUN_DEBUG=ON

  OFF — verbose VM debug logging (default OFF)

• FUN_WITH_CURL=ON

  OFF — enable CURL (libcurl) support (default OFF)

• FUN_WITH_JSON=ON

  OFF — enable JSON (json-c) support (default OFF)

• FUN_WITH_XML2=ON

  OFF — enable XML (libxml2) support (default OFF)

• FUN_WITH_PCRE2=ON

  OFF — enable PCRE2 (Perl-Compatible Regular Expressions) (default OFF)

• FUN_WITH_PCSC=ON

  OFF — enable PC/SC smart card (PCSC lite) support (default OFF)

• FUN_WITH_REPL=ON

  OFF — enable the interactive REPL (default OFF)

• FUN_WITH_SQLITE=ON

  OFF — enable SQLite (sqlite3) support (default OFF)

• FUN_WITH_INI=ON

  OFF — enable INI (iniparser) support (default OFF)

You can also set the default search path for the bundled stdlib with DEFAULT_LIB_DIR:

cmake -S . -B build -DDEFAULT_LIB_DIR="/usr/share/fun/lib" -DFUN_WITH_REPL=ON

If you encounter a CMake error such as:

CMake Error: Parse error in command line argument: FUN_WITH_JSON Should be: VAR:type=value

it means you passed a -D option without a value. Always use the form -DNAME=VALUE (e.g., -DFUN_WITH_JSON=ON).

SQLite example (optional feature)

SQLite support is optional and disabled by default. To build with it and run the example:

cmake -S . -B build -DFUN_WITH_SQLITE=ON
cmake --build build --target fun


sqlite3 ./database.sqlite < ./examples/data/database.sql


FUN_LIB_DIR="$(pwd)/lib" ./build/fun ./examples/sqlite_example.fun

XML example (optional feature)

XML support (via libxml2) is optional and disabled by default. To build with it and run the example:

cmake -S . -B build -DFUN_WITH_XML2=ON
cmake --build build --target fun


FUN_LIB_DIR="$(pwd)/lib" ./build/fun ./examples/xml_class_example.fun

Available VM builtins when built with -DFUN_WITH_XML2=ON:

• xml_parse(text: string) -> doc_handle (int > 0) or 0 on error
• xml_root(doc_handle: int) -> node_handle (int > 0) or 0 if missing
• xml_name(node_handle: int) -> string (node tag name)
• xml_text(node_handle: int) -> string (concatenated text of subtree)

Standard library wrapper (lib/io/xml.fun):

• class XML
  • parse(text: string): int (doc handle)
  • from_file(path: string): int (doc handle)
  • root(doc: int): int (node handle)
  • name(node: int): string
  • text(node: int): string

Example Fun code:

include <io/xml.fun>

xml = XML()
doc = xml.from_file("./examples/data/example.xml")
if (doc == 0)
  print("Failed to load XML file")
else
  root = xml.root(doc)
  print(xml.name(root))
  print(xml.text(root))

Notes:

• Handles are simple integers managed by the VM; nodes are owned by their document.
• This initial integration focuses on parsing and basic navigation. Attributes, children iteration, and XPath may be added later.

Install Fun to the OS (optional)

Not recommended during early development, but supported:

sudo cmake --build build --target install

After installation, FUN_LIB_DIR usually isn’t needed because libs are placed in the system default directory (e.g., /usr/share/fun/lib).

Usage

Run a script:

fun ./demo.fun

Table of contents

• Language overview and VM internals
• Command line interface and REPL
• Core types and operations
• Built-ins overview (what the VM provides)
• Standard library APIs
  • io.console
  • io.process
  • io.socket
  • io.thread
  • utils.datetime
  • regex
  • crypt (MD5, SHA-1/256/384/512)
  • encoding.base64
  • arrays, strings, maps helpers (hex, range)
• Extra libraries
  • JSON (via json-c)
  • CURL (via libcurl)
  • PCSC (PC/SC smart card)
  • SQLite (sqlite3)
• Examples reference

Language overview and VM internals

Fun compiles .fun source files to bytecode and executes them on a stack-based VM. Functions and methods push/pop their arguments and return values on a value stack.

High-level architecture:

• Front-end: parses .fun files, handles includes and constant folding, emits bytecode with debug markers (OP_LINE) used by tracing and REPL-on-error.
• VM core: runs a loop over opcodes (see src/bytecode.h). Values include numbers (integers), strings, arrays, maps, booleans (1/0), functions, and nil.
• Built-ins: I/O, strings, arrays, regex, date/time, OS, networking, threading, and optional JSON/PCRE2/CURL/PCSC/SQLite.

Selected VM concepts (non-exhaustive):

• Control flow: OP_JUMP, OP_JUMP_IF_FALSE, OP_RETURN
• Arithmetic/logic: OP_ADD/SUB/MUL/DIV, OP_MOD, OP_LT/LTE/GT/GTE, OP_EQ/NEQ, OP_AND/OR/NOT
• Stack helpers: OP_DUP, OP_SWAP, OP_POP
• Arrays: OP_MAKE_ARRAY, OP_INDEX_GET/SET, OP_LEN, OP_PUSH, OP_APOP, OP_INSERT/REMOVE, OP_SLICE
• Strings: OP_SUBSTR, OP_SPLIT, OP_JOIN, OP_FIND
• Maps: OP_MAKE_MAP; index ops shared with arrays
• Conversion/typing: OP_TO_NUMBER, OP_TO_STRING, OP_CAST, OP_TYPEOF, OP_UCLAMP/OP_SCLAMP
• Regex: OP_REGEX_MATCH/SEARCH/REPLACE (requires PCRE2 when built)
• Math: OP_MIN/MAX/CLAMP/ABS/POW, OP_RANDOM_SEED/RANDOM_INT
• Iteration helpers: OP_ENUMERATE, OP_ZIP
• OS/IO/network: sockets, files, processes, environment, threads, etc.
• Optional features: JSON (src/vm/json/), CURL, PCSC, SQLite (src/vm/sqlite/)

Error handling and debugging:

• Build with FUN_DEBUG=ON for verbose traces
• Run with –trace to print executed lines/opcodes
• Run with –repl-on-error to drop into an interactive REPL when a runtime error occurs

Command line interface and REPL

• Run a script: fun path/to/script.fun
• Common options: –trace, –repl-on-error (can be combined). REPL requires FUN_WITH_REPL=ON at build time.
• In trace/REPL-on-error modes, the VM annotates output with file:line and function names for easier debugging (see examples/debug_reporting.fun).

Core types and operations

Types:

• number: signed integer (with helpers for unsigned behavior)
• float: 64-bit IEEE-754 floating point
• string: immutable bytes; len(s), join, split, substr, find
• array: ordered list; len, push, apop, insert, remove, slice
• map: associative dictionary typically keyed by strings

• boolean: represented as 1 (true) or 0 (false); operators &&,

  , !

• nil: absence of value

Numeric variants:

• byte (uint8)
• Fixed width ints: int8/16/32/64 and uint8/16/32/64

Control flow:

• if/else, while; break, continue; range helpers in utils.range

Functions and classes:

• Define a function: fun name(args) …
• Define a class: class Name(constructor params) with method definitions fun method(this, …)
• _construct acts as the constructor if present; methods use explicit this

Exceptions:

• try { … } catch (e) { … } finally { … }
• Throwing/catching is defined by the spec; some runtimes may implement handling progressively. Examples: try_catch_finally.fun

Modules and includes:

• #include <path/to/module.fun> for libs under FUN_LIB_DIR
• #include “relative/path.fun” for local includes
• Namespacing via as: #include <utils/math.fun> as m; then call m.add(…)

Built-ins overview

Console and I/O:

• print(x) — prints value plus newline
• input(prompt) — read line from stdin

Strings and arrays:

• len(x), join(array, sep), split(text, sep), substr(text, start, len), find(text, needle)
• push(array, v), apop(array), insert(array, i, v), remove(array, i), slice(array, start, end)

Conversion and type:

• to_number(x), to_string(x), cast(value, typeName), typeof(x)
• uclamp(number, bits), sclamp(number, bits)

Math and random:

• min(a,b), max(a,b), clamp(x, lo, hi), abs(x), pow(a,b), random_seed(seed), random_int(lo, hiExclusive)

Bitwise (uint32 operations):

• band(a,b), bor(a,b), bxor(a,b), bnot(a)
• shl(a, s), shr(a, s)

Regex (requires PCRE2 when enabled):

• regex_match(text, pattern) -> 1/0
• regex_search(text, pattern) -> map { match, start, end, groups }
• regex_replace(text, pattern, repl) -> string

OS and processes:

• proc_run(cmd) -> { out: string, code: number }
• system(cmd) -> exit code
• env_get(name), env_set(name, value)

Networking and sockets:

• tcp_connect(host, port) -> fd (>0) or 0
• sock_send(fd, data) -> bytes or -1; sock_recv(fd, maxlen) -> string; sock_close(fd)
• tcp_listen(port, backlog) -> listen fd; tcp_accept(listenFd) -> client fd
• unix_connect(path) -> fd

Threads:

• thread_spawn(func, args) -> thread id; thread_join(id) -> return value

Date and time:

• time_now_ms(), clock_mono_ms(), date_format(ms, fmt)

JSON (optional):

• json_parse(text) -> value or nil
• json_stringify(value, prettyFlag) -> string
• json_from_file(path) -> value or nil
• json_to_file(path, value, prettyFlag) -> 1/0

PC/SC (optional):

• pcsc_establish() -> context id (>0) or 0
• pcsc_list_readers(ctx) -> array of reader names or nil
• pcsc_connect(ctx, readerName) -> handle id (>0) or 0
• pcsc_disconnect(handle) -> 1/0
• pcsc_transmit(handle, bytesArray) -> { data, sw1, sw2, code }

SQLite (optional):

• sqlite_open(path) -> handle (>0) or 0 on error
• sqlite_exec(handle, sql) -> rc (0 = SQLITE_OK)
• sqlite_query(handle, sql) -> array of row maps (string keys)
• sqlite_close(handle) -> nil

Note: Optional features depend on the CMake flags used when building.

Standard library APIs

The stdlib provides small wrappers around VM built-ins, typically organized in classes to avoid global name collisions and to offer sensible defaults.

io.console

Class Console (lib/io/console.fun):

• prompt(text) -> string
• ask(question) -> string
• ask_yes_no(question) -> 1/0 (y/yes vs n/no)

Example: examples/input_example.fun

io.process

Class Process (lib/io/process.fun):

• run(cmd) -> { out, code }
• run_merge_stderr(cmd) -> { out, code }
• system(cmd) -> number
• check_call(cmd) -> 1/0

Example: examples/process_example.fun

io.socket

Provides TcpClient, TcpServer, UnixClient (lib/io/socket.fun).

TcpClient:

• connect(host, port) -> 1/0; is_connected() -> 1/0
• send(data) -> bytes or -1; recv(maxlen) -> string; recv_all(chunk_size) -> string
• close()

TcpServer(port, backlog):

• listen() -> listen fd or 0; accept() -> client fd
• echo_once(maxlen) -> 1 when handled; serve_forever(maxlen) -> never returns
• close()

UnixClient:

• connect(path), is_connected(), send(data), recv(maxlen), close()

Examples: tcp_http_get.fun, tcp_http_get_class.fun, unix_socket_echo.fun, extra/tcp_echo_server_class.fun

io.thread

Class Thread (lib/io/thread.fun):

• spawn(func, args) -> thread id; join(id) -> return value
• Aliases: start(func, args), wait(id)

Examples: threads_demo.fun, thread_class_example.fun

utils.datetime

Class DateTime (lib/utils/datetime.fun):

• now_ms(), mono_ms(), format(ms, fmt), iso_now()

Example: datetime_basic.fun

regex

Class Regex (lib/regex.fun):

• match(text, pattern) -> 1/0
• search(text, pattern) -> { match, start, end, groups }
• replace(text, pattern, repl) -> string

Examples: regex_demo.fun, regex_procedural.fun

crypt

MD5 (lib/crypt/md5.fun) and SHA family (sha1/sha256/sha384/sha512) provide digest classes and helpers. Examples: md5_demo.fun, sha1_demo.fun, sha256_demo.fun, sha256_str_demo.fun, sha384_example.fun, sha512_demo.fun, sha512_str_demo.fun

encoding.base64

Module lib/encoding/base64.fun: base64_encode(string), base64_decode(string)

arrays, strings, maps helpers

• lib/arrays.fun — array helpers
• lib/strings.fun — string helpers (lower/upper, etc.)
• lib/hex.fun — bytes_to_hex, hex_to_bytes
• lib/utils/range.fun — numeric ranges
• lib/utils/math.fun and lib/math.fun — math helpers

Extra libraries

JSON (optional)

Build flag: -DFUN_WITH_JSON=ON; requires json-c.

VM API:

• json_parse(text) -> value or nil
• json_stringify(value, prettyFlag) -> string
• json_from_file(path) -> value or nil
• json_to_file(path, value, prettyFlag) -> 1/0

Stdlib wrapper:

• class JSON (lib/io/json.fun) — convenience methods mirroring the VM API.

Example:

include <io/json.fun>

j = JSON()
data = j.parse('{"name":"Fun","year":2026,"ok":true,"tags":["vm","lang"]}')
print(typeof(data))  // map
print(data["name"])

ok = j.to_file("./tmp/out.json", data, 1)  // pretty = 1
print("saved:", ok)

Notes:

• JSON types map to Fun types: object -> map, array -> array, string -> string, number -> number/float, true/false -> 1/0, null -> nil.
• When writing, prettyFlag=1 enables pretty printing.

cURL (optional)

Build flag: -DFUN_WITH_CURL=ON; requires libcurl.

VM API:

• curl_get(url) -> string (empty string on error)
• curl_post(url, body) -> string (empty string on error)
• curl_download(url, path) -> 1/0

Stdlib wrapper:

• None (call built-ins directly). See examples in examples/extra/.

Example:

url = "https://httpbin.org/get"
resp = curl_get(url)
if (len(resp) == 0)
  print("GET failed")
else
  print(substr(resp, 0, 60), "...")

Examples:

• curl_get_json.fun, curl_post.fun, curl_download.fun

PCSC (optional)

Build flag: -DFUN_WITH_PCSC=ON; requires PC/SC (pcsc-lite).

VM API:

• pcsc_establish() -> context id (>0) or 0
• pcsc_list_readers(ctx) -> array of reader names or nil
• pcsc_connect(ctx, readerName) -> handle id (>0) or 0
• pcsc_disconnect(handle) -> 1/0
• pcsc_transmit(handle, bytesArray) -> { data: array, sw1: number, sw2: number, code: number }

Stdlib wrapper:

• class PCSC (lib/io/pcsc.fun) — higher-level helpers for listing readers, connecting, and APDU I/O.

Example:

include <io/pcsc.fun>

sc = PCSC()
ctx = pcsc_establish()
if (ctx == 0)
  print("PCSC not available")
else
  readers = pcsc_list_readers(ctx)
  if (readers == nil || len(readers) == 0)
    print("No readers found")
  else
    h = pcsc_connect(ctx, readers[0])
    if (h)
      // Example APDU: GET RESPONSE (illustrative only)
      res = pcsc_transmit(h, [0x00, 0xC0, 0x00, 0x00, 0x00])
      print("SW:", res["sw1"], res["sw2"], "code:", res["code"]) 
      pcsc_disconnect(h)

Examples:

• examples/extra/pcsc_example.fun, examples/extra/pcsc_demo.fun

Notes:

• Smart card operations depend on the reader, card, and drivers installed. Ensure pcscd/service is running.

SQLite (optional)

Build flag: -DFUN_WITH_SQLITE=ON; requires sqlite3 development headers.

VM API:

• sqlite_open(path) -> handle (>0) or 0
• sqlite_exec(handle, sql) -> rc (0 = SQLITE_OK)
• sqlite_query(handle, sql) -> array of maps (columns as string keys)
• sqlite_close(handle) -> nil

Result mapping notes:

• INTEGER -> number
• FLOAT -> number (floating point)
• TEXT -> string
• NULL -> nil
• BLOB is currently not returned (mapped to nil)

Example flow (examples/sqlite_example.fun): 1) h = sqlite_open(“./todo.sqlite”) 2) rows = sqlite_query(h, “SELECT id, title, done, created_at FROM tasks ORDER BY id;”) 3) rc = sqlite_exec(h, “INSERT INTO tasks (title, done) VALUES (‘Try Fun + SQLite’, 0);”) 4) rows2 = sqlite_query(h, “SELECT count(*) AS cnt FROM tasks;”) 5) sqlite_close(h)

INI (optional)

Build flag: -DFUN_WITH_INI=ON; requires iniparser (>= 4.2.6).

VM API:

• ini_load(path) -> handle (>0) or 0 on error
• ini_free(handle) -> 1/0
• ini_get_string(handle, section, key, default) -> string
• ini_get_int(handle, section, key, defaultNumber) -> number
• ini_get_double(handle, section, key, defaultFloat) -> float
• ini_get_bool(handle, section, key, defaultBool) -> 1/0
• ini_set(handle, section, key, valueString) -> 1/0
• ini_unset(handle, section, key) -> 1/0
• ini_save(handle, path) -> 1/0

Notes:

• Section/key may be looked up flexibly (“section:key” and “section.key”).
• Always free handles with ini_free when done.

Example:

h = ini_load("./examples/data/example.ini")
if (h == 0)
  print("Failed to load INI")
else
  host = ini_get_string(h, "server", "host", "localhost")
  port = ini_get_int(h, "server", "port", 8080)
  print("server:", host, port)
  ok = ini_set(h, "server", "host", "127.0.0.1")
  if (ok)
    ini_save(h, "./tmp/updated.ini")
  ini_free(h)

XML (optional)

Build flag: -DFUN_WITH_XML2=ON; requires libxml2.

VM API:

• xml_parse(text) -> doc_handle (>0) or 0 on error
• xml_root(doc_handle) -> node_handle (>0) or 0 if missing
• xml_name(node_handle) -> string
• xml_text(node_handle) -> string

Stdlib wrapper:

• class XML (lib/io/xml.fun)
  • parse(text): int
  • from_file(path): int
  • root(doc): int
  • name(node): string
  • text(node): string

Example:

include <io/xml.fun>

xml = XML()
doc = xml.from_file("./examples/data/example.xml")
if (doc == 0)
  print("Failed to load XML file")
else
  root = xml.root(doc)
  print(xml.name(root))
  print(xml.text(root))

Notes:

• Handles are integers managed by the VM; nodes belong to their document.

PCRE2 / Regex (optional)

Build flag: -DFUN_WITH_PCRE2=ON; requires PCRE2 (8-bit API).

VM API:

• regex_match(text, pattern) -> 1/0
• regex_search(text, pattern) -> map { match, start, end, groups }
• regex_replace(text, pattern, repl) -> string

Stdlib wrapper:

• class Regex (lib/regex.fun) providing match/search/replace helpers.

Examples: regex_demo.fun, regex_procedural.fun

Notes:

• Patterns use PCRE2 syntax.

REPL (optional)

Build flag: -DFUN_WITH_REPL=ON.

Description:

• Enables the interactive Read–Eval–Print Loop and the –repl-on-error mode. No additional VM API functions; this feature is part of the executable.

Examples reference

You can run examples without installing by pointing FUN_LIB_DIR to the repository lib directory:

FUN_LIB_DIR=”$(pwd)/lib” ./build/fun examples/.fun

Highlights (not exhaustive):

• arrays.fun, arrays_advanced.fun, arrays_iter.fun — array operations
• booleans.fun, boolean_decl.fun — boolean basics
• builtins_conversions.fun, builtins_extended.fun — conversions and math helpers
• builtins_maps_and_more.fun — maps and indexing
• byte_for_demo.fun — bitwise operations
• class_constructor.fun, classes_demo.fun, inheritance_demo.fun — classes
• datetime_basic.fun — date/time utilities
• debug_reporting.fun, repl_on_error.fun — tracing and REPL-on-error
• exit_example.fun — exit codes
• expressions_test.fun — operators
• file_io.fun, file_print_for_file_line_by_line.fun — file I/O
• for_range_test.fun — numeric ranges
• functions_test.fun — functions and higher-order usage
• have_fun.fun — quick sanity check
• if_else_test.fun — branching
• include_lib.fun, include_local.fun, include_namespace.fun — includes and namespacing
• interactive/input_example.fun — console input
• extra/json_showcase.fun — JSON usage
• extra/curl_get_json.fun, extra/curl_post.fun, extra/curl_download.fun — HTTP via CURL
• loops_break_continue.fun, nested_loops.fun, while_test.fun — loops
• crypto/md5_demo.fun, crypto/sha1_demo.fun, crypto/sha256_demo.fun, crypto/sha256_str_demo.fun, crypto/sha384_example.fun, crypto/sha512_demo.fun, crypto/sha512_str_demo.fun — hashing
• objects_basic.fun, objects_more.fun — map/object patterns
• os_env.fun — environment variables
• extra/pcsc_example.fun — smart card demo
• process_example.fun — running external commands
• regex_demo.fun, regex_procedural.fun — regex usage
• stdlib_showcase.fun — tour through stdlib
• strings_test.fun — string operations
• tcp_http_get.fun, tcp_http_get_class.fun — TCP client demos
• thread_class_example.fun, threads_demo.fun — threading
• try_catch_finally.fun, try_catch_with_error.fun — error handling
• typeof.fun, typeof_features.fun — types and casting
• type_safety.fun, type_safety_fails.fun — type safety
• types_integers.fun, signed_ints.fun, uint_types.fun — integers
• unix_socket_echo.fun — UNIX domain sockets
• extra/sqlite_example.fun — SQLite usage

Notes:

• Some examples rely on optional features (JSON, CURL, PCSC, SQLite) and degrade gracefully when disabled.

Internals notes (selected)

JSON: src/vm/json/* wraps json-c. OP_JSON_PARSE and friends convert json_object to Fun values and back; stdlib JSON class adds ergonomics.

PCSC: The VM interfaces with pcsc-lite/WinSCard and returns maps with data and status words. The stdlib wrapper handles absent hardware defensively.

SQLite: src/vm/sqlite/* implements open/exec/query/close using a simple handle registry. Query prepares a statement, steps rows, maps columns by name to values, and returns an array of row maps.

Development

This project follows Semantic Versioning. Commit messages include the version (e.g., 1.2.3). Version bumps are made in CMakeLists.txt for code changes; documentation-only commits include the current version in the message but do not bump it.

Development systems

• GNU/Linux (glibc, musl), FreeBSD (Clang), Windows (Cygwin + GCC). Other Unix-like systems likely work but are untested.

Contributing and further reading

• Browse lib/ for stdlib APIs (files often document their own interfaces)
• src/bytecode.h lists supported opcodes; implementations live under src/vm/
• examples/ are the best starting point to learn by doing

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