Capybara is a single-page web application that allows interactive graph reduction of lambda expressions. It is designed to make the understanding and comparison of different evaluation strategies (Call-By-Name, Call-By-Value and Call-By-Need) easier.
The implementation is completely written in Haskell. The frontend is programmed using miso. Bootstrap is used to style the interface. The graphs are displayed using SVG.
This is a project for the lecture Fortgeschrittene Funktionale Programmierung at LMU in WS21/22.
The interface is rather self-explanatory (hopefully XD), but I will still list the features here.
The user can type a lambda expression in the input box and choose an evaluation strategy. For the lambda symbol, you can simply type a backslash, it will be automatically turned into λ when evaluating.
Variable names can be letters, digits, underscores and prime symbols. But they should start with a letter.
Supercombinators without parameters are also supported. Some supercombinators are already provided. You can add your own ones of course.
Line comments start with --.
When there are errors in your term or supercombinator definitions, the corresponding box will light up and an error message is displayed.
Bound variables will be renamed by attaching subscripts to them.
When graphs are displayed, you can click on the ◄ and ► buttons or press left and right arrow keys to go through them. The reducible expression of every beta-reduction step is marked in blue. Unfolding the definition of a supercombinator is considered one step of reduction.
No problem if you have a Linux System! (Unfortunately there is currently a bug in miso, which makes building on macOS impossible.)
You first need to install Nix on your machine.
sh <(curl -L https://nixos.org/nix/install) --no-daemon
Next step is to install Cachix to make the build process faster.
nix-env -iA cachix -f https://cachix.org/api/v1/install
cachix use miso-haskell
I assume you have already cloned this repository. Then just go to the directory. I have already prepared a Makefile for you. A simple make should do the trick.
Now we are all set! Simply open result-webpage/index.html with your favourite browser, Capybara will take it from there :)
Because I love them! They are the largest rodents in the world. But what really makes them stand out is how chilled they are. If you have never heard of this animal, check out this video. Don't they brighten you up?
I hope my little application could also bring you some joy and make you feel at ease with this fairly abstract topic. Have fun!