A friendly lisp with a focus on interactivity and collaboration
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Trifle Lisp Build Status Coverage Status Requirements Status Scrutinizer Code Quality Code Health

A sweet and friendly lisp

Current status: Only a basic interpreter implemented. Please see the docs to see what's available.

Table of Contents


Trifle is:

Prototypes to Products

Trifle will let users write a quick and dirty script that uses global variables and mutable state everywhere. Trifle will also let users unit test, lint, type check and add contracts to their code. Users will be able to choose how much safety they want.


Trifle actively avoids including a standard library. Instead, it opts for including a package manager so that each package can evolve separately. Packages use semantic versioning and declare their dependencies.

Trifle takes an 'open implementation' approach. Wherever possible, functionality is built on top of the core, to encourage experimentation and to minimise the amount of non-Trifle code users must read. Where non-essential functions are written in RPython for performance, an equivalent Trifle implementation will be included.

In spite of encouraging libraries to live outside of the core, the Trifle community will promote a set of defaults, so new users have a standard set of tools to get started with.

Influences: Scheme's small core, CLOS as a library, npm, Smalltalk


The Trifle standard is not set in stone. Major versions may break backwards compatibility. The featureset will change over time to maximise readability and expressiveness.

Influences: Python 3 cleaning up semantics


Trifle seeks to make code understandable, when looking at both individual pieces of code or the high-level overview. Trifle supports docstrings and cross-references to package documentation.

Names are chosen to be clear, self-explanatory and minimally abbreviated. Code should be concise through well-chosen abstraction instead of very short names.

Influences: Python's readability, Python doctests, Elisp docstrings


Trifle seeks to be clearly and thoroughly documented. Documentation should include examples wherever possible. Each page in the documentation should have a small and well-defined purpose, with each function on a separate page. Users should be able to leave comments on the documentation for later readers.

Influences: PHP docs, clojuredocs


Trifle features closures, unhygenic macros and reader macros.

Influences: Common Lisp


Trifle re-considers traditional lisp features. There is no built-in Cons cell (lists are vectors). Some common functions have been renamed (car) and others have different behaviours to other lisps (last returns the last element in a list). Trailing parentheses are encouraged to aid readability.

Influences: Clojure


Trifle places a high value on code readability for users who have programmed in other languages but are new to Trifle (or lisps in general). It tries to use familiar terminology and avoids abbreviations.


The Trifle community strives to be friendly and helpful. We have a code of conduct that applies to all official Trifle communication channels.

The language and its libraries are developed in the open on GitHub. The package manager allows different forks of libraries to coexist.

Influences: Rust/CoffeeScript on GitHub

Human Focused Performance

When Trifle features have to decide between helping the user and helping the machine, Trifle sides with the user every time.

That said, Trifle seeks to provide a fast implementation for this human-focused feature set. We do this by implementing the intepreter in RPython, giving us a JIT with little work. The language provides also provides opt-in TCO.

Influences: Pypy, Scheme

Missing Features

A new programming language should really have a good answer to 'how do I run it in the browser?', 'how do I scale it to multiple cores?' and 'how do I interface with C code?'. Since this is an experimental language, we are cheerfully ignoring these questions for now.

Release History

v0.12 (not yet released)

Fixed VERSION which hadn't been updated since version 0.9.


Added hashmaps, e.g. {1 2, 3 4}. Please see the docs for a full description. Added the functions hashmap?, get-key, get-items and set-key!.

It's now an error to repeat parameters (e.g. calling (lambda (x x) #null)) in a function or macro.

Commas are now treated as whitespace, so (1, 2, 3) is a valid list literal.

Fixed equality bugs in fractions and keywords. Fixed an equality bug with booleans returned from defined?.


Fractions: Fraction numerator and denominators are now arbitrary size and never overflow. They were previously limited to 32 bits.

Integers: Integers are now arbitrary size and never overflow. They were also previously limited to 32 bits. Note that indexing into sequences above 2 ** 32 will not yet work.

Parsing: Fixed a bug with parse where it sometimes returned a list of one error instead of throwing an error on invalid input.


Shell: Added a pure Trifle shell program: shell.tfl. The interpreter itself now only accepts file paths or -i snippets.

Exceptions: Added the function exception-type. Fixed a bug where arity-error or stack-overflow couldn't be caught if thrown by a built-in function or macro.

Emacs mode: trifle-mode now understand how to indent Trifle code.

Sequences: Added a separate docs page for sequences. Added the function sequence?.

Evaluation: Fixed an interpreter crash on evaluating an empty list.

Macros: Fixed an interpreter crash on throwing exception during macro expansion.

Literals: Repeatedly evaluating list, string and bytestring literals now returns a fresh copy every time. This fixes programs like the following:

(function empty-string () "")
(set! x (empty-string))
(append! x 'a')
(empty-string) ; Used to return "a"!

File handles: Added the function flush!.

Symbols: _ is now a legal symbol. Symbols may now include uppercase characters. Added the function symbol?.

Strings: Added the constant VERSION.

Loops: Added the macro loop for infinite loops.

Generic functions: Added the function printable.


Exceptions: Added an exception system. All errors thrown by all built-ins are now exceptions. Added the special expression try and the built-in functions throw and message. Defined the built-in exceptions error (the base exception), stack-overflow, no-such-variable, parse-failed, lex-failed, value-error, wrong-type, wrong-argument-number, division-by-zero, file-not-found, and changing-closed-handle.

Sample programs: Added a factorial program.

Errors messages: Improved wording on calling non-callables. Arity errors now include more information on the number of arguments received.

Booleans: if now always requires three arguments. Added when for when users don't care about else. unless was renamed to when-not.

Bytestrings: The lexer now understand all legal escapes in bytestring literals. Fixed a lexing bug with consecutive bytestring literals.

Numbers: >, >=, < and <= now work with fractions too.

Stack: Overflowing the stack now raises an error rather than crashing the interpreter.


Bytestrings: Added the function bytestring?.

Sequences: Added the functions empty, empty?, copy, join! and join.

Booleans: Removed truthy?. if and while now require booleans for their conditions, instead of just truthy values.

Docs: Added a 'generic functions' section.

Generic functions: Added identity.

Numbers: Added a fraction type. Added support for fractions to +, -, * and /.

Errors: Various minor improvements to error messages to be more consistent and helpful.


Installation: It's now possible to do make install and make uninstall to install a Trifle interpreter.

Prelude: The interpreter is now more helpful when it can't find the prelude (instead of crashing).

Escape sequences: Trifle now supports backslash escape sequences in strings and characters, so '\'' and "they said \"hello\"" are now legal literals.

Booleans: Added short-circuiting macros and and or.

Numbers: Added the functions >, >= and <=.

Equality: Fixed a bug where character equality considered characters to not be equal? if they are not same?. Identical characters are now equal, so (equal? 'a' a') is #true.

Lists: Added the built-in function insert!. append! and push! have been rewritten as Trifle functions in the prelude. Added the function filter.

Special expressions: Special expressions are no longer first class values. You can no longer do things like (set! foo if) or ((lambda (f x y) (f x y)) if #true 1). This is intended to make static analysis more tractable.


Strings: Strings are now sequences of characters and all sequence functions (get-index, set-index!, append!, push!, length, map, rest, first, second, third, fourth, fifth, and last) can be called on them. Also added a string? function.

Character: Added a character datatype. Added a character? function.

Errors: Fixed error messages in length and parse which incorrectly stated the expected type.

Docs: Fixed some minor mistakes.


Bytestrings: 'Bytes' are now called bytestrings, and literal bytestrings are recognised by the interpreter (e.g. #bytes("abc")). Bytestrings are now mutable.

Sequences: Trifle now has the concept of a 'sequence', which is a mutable ordered datatype. Lists and bytestrings are both sequences. The expressions length, get-index, set-index!, first, second, third, fourth, fifth, append! and map now handle sequences. Added the function last. The macro for-each now handles sequences.

Lists: Added the functions list?, rest and range.

Booleans: The literal boolean syntax has changed to #true and #false. Added the macros unless and case. truthy? has moved to the prelude. The values "", #bytes("") and 0.0 are now considered to be falsey.

Null: The literal null syntax has changed to #null.

Strings: print is now print!.

Numbers: Added the functions zero?, mod and div.

Errors: get-index, set-index! and push! now use arity errors and value errors where appropriate. Fixed some error wording issues with open.

IO: Added an exit! function.

Evaluation: quote is now much stricter about arguments passed to unquote and unquote* instead of silently ignoring them. call can now handle macros as well as functions.

Interpreter: When given the path to a function, the result of the program is no longer automatically printed. Use print! in your program instead.

Editing: There is now an Emacs major mode available, see Getting Started.

Sample programs: Added hello world, fibonacci and fizzbuzz sample programs.

Licensing: Trifle is now dual-licensed under MIT and Apache 2.0. This will not change again.


File handles: Added a file handle datatype, and added the functions open, close, read and write!.

Bytes: Added a bytes datatype, and added the function decode.

Strings: Fixed many unicode bugs. Added the function encode.

Equality: Added the function equal?. same? no longer treats numbers specially, so it's now possible for two numbers to be equal? but not same?. same? now throws ArityError if given the wrong number of arguments.

Built-in functions: String representations have been improved.

Docs: The documentation has been grouped into more logical sections, and datatypes have been separated from other documentation pages.


Numbers: This release adds floats, and makes parsing stricter. 123foo was previously a symbol, it is now a syntax error. The function / and macro dec! have been added.

Evaluation: The functions call, parse, eval and defined? have been added.

Macros: Fixed a bug with macro where it reported the wrong argument as incorrect. Macros now always require a body.

Errors: Many built-in functions have had their error messages improved, and raise an arity error (not a type error) on the wrong number of arguments.

Documentation: Docs have been improved, and function and let have gained docmentation. We've cleared up the difference between functions (arguments always evaluated) and special expressions (does not evaluate some arguments).

I/O: The function input (read a line from stdin) has been added.

Workflow: We now use coveralls to measure code coverage on Python code for every checkin. Note we don't have any facility for measuring coverage on Trifle code yet.


This release includes integers, lists, booleans, strings (though you can do little with strings currently), symbols, loops, functions, closures and macros.