LCI is an interpreter for the lambda calculus. It supports many advanced features such as recursion, user-defined operators and multiple evaluation strategies, all based on the pure calculus. It is free software licenced under the GNU General Public Licence (GPL).

Try it

LCI is a command line program, but it can also run in a browser via WebAssembly. Try it below or open the fullpage demo.

Features

LCI can be considered a small (but powerfull) functional progamming language based on the pure lambda-calculus. Its features include:

• Aliases of lambda terms (that is named functions).
• Integers
  • An arbitrary encoding can be used by defining 0, Succ, Pred, IsZero.
  • Church encoding is used by default in .lcirc.
  • Scott encoding is also available (uncomment to use it)
• Recursion. Self-references of aliases are expanded during execution. LCI can also automatically convert recursive terms to non-recursive ones using a fixed point combinator.
• User-defined operators. The user can declare a new operator with a certain precedence and associativity and define it in lambda calculus. Many common operators (eg. integer, logic and list operations) are pre-defined in .lcirc and are available by default.
• List syntax. [a,b,c] is parsed as a:b:c:Nil (: and Nil are defined in .lcirc).
• Let syntax. let x = M in N is parsed as (\x.N) M.
• Multiple evaluation strategies. Call-by-name and call-by-value can coexist in the same program.
• Human-readable display of terms: for example church numerals are displayed as numbers and lists using the [a,b,c] notation.
• Tracing of execution.
• File interpretation as well as interactive usage.
• A library of pre-defined functions (.lcirc).

All features are implemented in the pure lambda calculus. To demonstrate them, there is an implementation of the N-Queens problem (queens.lci) in a way that reminds of Haskell syntax.

Install

From source

The latest version is available here. To install extract the archive, cd to that directory and run:

cmake -B build
cd build && make
sudo make install

This will install the lci executable in /usr/local/bin and .lcirc, queens.lci in /usr/local/share/lci. You can install then in a different location by passing -DCMAKE_INSTALL_PREFIX=<dir> to cmake.

You can also checkout the code from github.

Using Homebrew on OSX

Install Homebrew and run:

brew install lci

Binaries for Windows

Windows binaries are available here. Simply extract and run the lci executable.

Building for WebAssembly

The browser version can be built with emscripten. You first need to build make_dparser with a normal build, then build again with emcmake. The build is created under build/html/dist.

mkdir build && cd build
cmake ..
make make_dparser

rm CMakeCache.txt
emcmake cmake ..
emmake make

Documentation

LCI’s documentation covers most of the program’s features and related lambda-calculus concepts.

Contributing

If you found a bug please report it. Also feel free to send pull requests, or suggest features.