PNLC

A small functional language

PNLC consists of two components:

1. An interpreter for a lazily-evaluated untyped λ‑calculus augmented with I/O.
2. A prelude that bootstraps the calculus into a practical language.

The Interpreter

A PNLC program is a λ‑term written in prefix notation. Prefix notation simplifies parsing and obviates syntax for let bindings or where clauses. The grammar is specified in grammar.bnf. Roughly speaking,

<term> ::= "." <term> <term> ; application
         | "\\" <var> <term> ; abstraction
         | <var>             ; variable

For example, λf.(λx.f(xx))(λx.f(xx)) is written \f .\x .f .x x \x .f .x x. There is no syntactic sugar apart from # comments. Binders shadow in the usual way, so \x \x x is α‑equivalent to \x \y y.

A well-formed program evaluates to a data structure that describes its I/O behavior, which the runtime then interprets. Conceptually it has the following shape:

data Prog = Exit
          | Err
          | Get (Maybe Bool -> Prog)
          | Put Bool Prog
          | Eput Bool Prog
          | Dump Term Prog

Operationally, the variables $exit, $err, $get, $put, $eput, $dump may occur free in a program’s λ‑term and are given the following reduction semantics:

Top-Level Term	Reduction
$exit	Terminate normally.
$err …	Discard any arguments and abort execution.
.$get cont	Read bit from stdin and reduce to .cont bit. A bit of \s \n n means EOF, \s \n .s \t \f t means one, \s \n .s \t \f f means zero.
..$put bit cont	Write bit to stdout and reduce to cont. A bit of \t \f t means one, \t \f f means zero.
..$eput bit cont	Write bit to stderr and reduce to cont. A bit of \t \f t means one, \t \f f means zero.
..$dump term cont	Dump an implementation-defined representation of term to stderr and reduce to cont.

The Prelude

The prelude defines data types like booleans, integers, pairs, optionals, lists, characters, strings; type classes like monoids, functors, monads, comonads, foldables; type class instances like monoids under addition, total ordering of lists, the stream functor, the I/O monad; and more.

Arguably, the prelude is the language, because without it you’re left with little more than a λ‑calculus interpreter. Much of it is inspired by Haskell’s base library and Miranda’s standard environment. As a first step in becoming familiar with it you might study the example programs included—though not the “naked” ones, because they’re designed not to depend on the prelude.

Examples

First, compile the interpreter:

make bin/pnlc

Then, run any of the example programs:

bin/pnlc examples/no-op\ naked.pnlc
bin/pnlc examples/hello\ world\ naked.pnlc
bin/pnlc examples/bit-cat\ naked.pnlc
bin/pnlc prelude.pnlc examples/no-op.pnlc
bin/pnlc prelude.pnlc examples/hello\ world.pnlc
bin/pnlc prelude.pnlc examples/bit-cat.pnlc
bin/pnlc prelude.pnlc examples/chr-cat.pnlc
bin/pnlc prelude.pnlc examples/greeting.pnlc
bin/pnlc prelude.pnlc examples/truth-machine.pnlc
bin/pnlc prelude.pnlc examples/reverse.pnlc
bin/pnlc prelude.pnlc examples/rot13.pnlc
bin/pnlc prelude.pnlc examples/quine.pnlc
bin/pnlc prelude.pnlc examples/yin-yang.pnlc
bin/pnlc prelude.pnlc examples/rule\ 110.pnlc
bin/pnlc prelude.pnlc examples/fizzbuzz.pnlc
bin/pnlc prelude.pnlc examples/hanoi.pnlc
bin/pnlc prelude.pnlc examples/ack.pnlc
bin/pnlc prelude.pnlc examples/fib\ dec.pnlc
bin/pnlc prelude.pnlc examples/fib\ bin.pnlc
bin/pnlc prelude.pnlc examples/collatz\ dec.pnlc
bin/pnlc prelude.pnlc examples/collatz\ bin.pnlc
bin/pnlc io\ hook.pnlc prelude.pnlc examples/hello\ world.pnlc
bin/pnlc io\ hook.pnlc prelude.pnlc examples/bit-cat.pnlc

# PNLC self-interpreter demos
bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc
bin/pnlc prelude.pnlc examples/pnlc\ metacircular.pnlc
bin/pnlc prelude.pnlc examples/pnlc\ shallow.pnlc
cat examples/no-op\ naked.pnlc         | bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc
cat examples/hello\ world\ naked.pnlc  | bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc
cat examples/bit-cat\ naked.pnlc nul - | bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc
cat prelude.pnlc examples/fib\ dec.pnlc       | bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc
cat prelude.pnlc examples/ack.pnlc            | bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc
cat prelude.pnlc examples/hanoi.pnlc          | bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc
cat prelude.pnlc examples/fizzbuzz.pnlc       | bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc
cat prelude.pnlc examples/greeting.pnlc nul - | bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc
cat prelude.pnlc examples/pnlc\ definitional.pnlc nul examples/no-op\ naked.pnlc         | bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc
cat prelude.pnlc examples/pnlc\ definitional.pnlc nul examples/hello\ world\ naked.pnlc  | bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc
cat prelude.pnlc examples/pnlc\ definitional.pnlc nul examples/bit-cat\ naked.pnlc nul - | bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc
cat io\ hook.pnlc examples/hello\ world\ naked.pnlc  | bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc
cat io\ hook.pnlc examples/bit-cat\ naked.pnlc nul - | bin/pnlc prelude.pnlc examples/pnlc\ definitional.pnlc

# regex engine demos
bin/pnlc prelude.pnlc examples/regex.pnlc
cat examples/re/comment.re pnlc.c                  | bin/pnlc prelude.pnlc examples/regex.pnlc
cat examples/re/strlit.re  pnlc.c                  | bin/pnlc prelude.pnlc examples/regex.pnlc
cat examples/re/utf-8.re   README.md               | bin/pnlc prelude.pnlc examples/regex.pnlc
cat examples/re/divby3.re  examples/rule\ 110.pnlc | bin/pnlc prelude.pnlc examples/regex.pnlc
cat examples/re/ipv4.re    examples/regex.pnlc     | bin/pnlc prelude.pnlc examples/regex.pnlc
cat examples/re/hexrgb.re  examples/regex.pnlc     | bin/pnlc prelude.pnlc examples/regex.pnlc

# Brainfuck interpreter demos
bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/pnlc.bf                 | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/bell.bf                 | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/ascii.bf                | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/cat.bf bang -           | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/reverse.bf bang -       | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/beaver.bf               | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/clear.bf                | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/head.bf bang -          | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/strip.bf bang -         | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/truth-machine.bf bang - | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/bin2text.bf bang -      | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/text2bf.bf bang -       | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/fib.bf                  | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/squares.bf              | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/thuemorse.bf            | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/sierpinski.bf           | bin/pnlc prelude.pnlc examples/brainfuck.pnlc
cat examples/bf/siercarpet.bf           | bin/pnlc prelude.pnlc examples/brainfuck.pnlc