Flurry

This is a Haskell implementation of the Flurry esoteric programming language. Flurry is a stack-based functional language with heavy inspiration from Brain-Flak.

Description

The main data type in Flurry is the expr. An expr is a function that takes a single input of type expr and produces an output of type expr. This function may modify the stack in the process (you can think of it as (expr, stack) -> (expr, stack) if you prefer).

The stack contains values of type expr. Attempting to pop from an empty stack will instead return the I combinator (λx. x).

The only syntactically relevant characters in a Flurry program are ()[]{}<>, and they must always be balanced. These expressions evaluate to an expr, and possibly modify the stack.

Here are the four nilads:

• [] evaluates to the height of the stack as a Church numeral.
• () evaluates to the K combinator (λxy. x).
• <> evaluates to the S combinator (λxyz. xz(yz)).
• {} pops the stack.

Here are the four monads:

• [...] does nothing and evaluates to ... (acting as a grouping operator).
• (...) pushes ... to the stack, and also returns it.
• <abc> evaluates to a ￮ b ￮ c (where ￮ represents function composition).
• {...} returns a function that evaluates ... with its argument on top of the stack (so {{}} is the identity function, for example).

Input and output are encoded as Church numerals on the stack. If any stack elements are not Church numerals, they are ignored.

Snippets

Duplicate the top stack value:

(({}))

Return the iota combinator (λx. xSK):

{{}<>()}

Return the sum of the top two stack values (assuming they are Church numerals):

{}[<><<>()>]{}

Return the product of the top two stack values (assuming they are Church numerals):

<{}{}>

Using the interpreter

You can compile the interpreter with

$ ghc -o target/Flurry -outputdir target src/*.hs

This will create a directory called target containing an executable called Flurry. You can run a program with:

$ echo "(<{}{}>)" > code.flr
$ target/Flurry -inn code.flr 10 20
200

You can also pass a program directly:

$ target/Flurry -inn -c "(<><<>()>({}))" 99
99 100

The first argument (-inn in this example) specifies IO behavior with 3 characters:

• The first character describes what happens to the stack after the program is finished:
  • i means to output them as integer values.
  • b means to output them as byte values.
  • d means to output them as integer values to STDERR.
  • n means to ignore them.
• The second character describes what happens to the return value of the program:
  • i means to output it as an integer value.
  • d means to output it as an integer value to STDERR.
  • n means to ignore it.
• The third character describes how STDIN is used to initialize the stack:
  • i means the contents of STDIN are interpreted as integers.
  • b means the contents of STDIN are interpreted as their byte values.
  • n means the contents of STDIN is ignored.

The default IO configuration is -ini if the code was passed in a file, and -ddn if the code was passed with -c.