modal/modal.py

"""
   *------------------------------------------------------------*
   |modal.py : An Implementation of Modal in Python             |
   |                                                            |
   |This is an implementation of Modal, a programming language  |
   |based on term rewriting. This particular implementation     |
   |is based on the idea of cyclic delimited string rewriting   |
   |using a central queue and a dictionary of variable bindings.|
   |                                                            |
   |© 2019-2024 wryl, Paradigital                               |
   *------------------------------------------------------------*
"""

# `sys`  for I/O and arguments.
# `time` for measuring the runtime of the interpreter.
import sys, time

# A decorator intended for measuring the runtime of a given function.
def measure(function):
    def measurement(*arguments):
        start        = time.time()
        result       = function(*arguments)
        end          = time.time()
        milliseconds = (end - start) * 1000.0
        print("\nTook {:.3f}ms".format(milliseconds))
        return result
    return measurement

# Enqueue an item by appending it to the queue.
def enqueue(queue, item):
    return queue + item

# Dequeue an item by slicing the queue. The last item is the head.
def dequeue(queue, length=1):
    if length > len(queue):
        return queue
    return queue[length:]

# Get the item(s) at the head of the queue.
def peek(queue, length=1):
    if length > len(queue):
        return None
    if length == 1:
        return queue[0]
    return queue[:length]

# Roll/cycle the queue by a certain amount by slicing.
# This dequeues and enqueues a number of items, "cycling" the queue.
def roll(queue, length=1):
    if length > len(queue):
        return queue
    return queue[length:] + queue[:length]

# Seek to a certain position in the queue by repeatedly rolling it.
def seek(queue, pattern):
    if pattern not in queue:
        return queue
    while peek(queue, len(pattern)) != pattern:
        queue = roll(queue)
    return queue

# Extract a delimited fragment (subtree) from the queue.
def extract(queue):
    results = []
    depth = 0
    for element in queue:
        if element[0] == "SRT":
            return []
        if element[0] == "INC":
            depth = depth + 1
        if element[0] == "DEC":
            if depth == 0:
                return results
            depth = depth - 1
        results.append(element)
        if depth == 0:
            return results
    return results

# Generate a list of variable bindings from the current queue and a pattern.
def match(pattern, queue, context=None):
    if context == None:
        context = {}
    if peek(queue) == None:
        return context
    for element in pattern:
        if element[0] == "VAR":
            variable = element[1]
            value    = extract(queue)
            if variable in context:
                if context[variable] != value:
                    return None
                queue = dequeue(queue, len(context[variable]))
            else:
                if len(value) == 0:
                    return None
                context[variable] = value
                queue             = dequeue(queue, len(context[variable]))
        elif element != peek(queue):
            return None
        else:
            queue = dequeue(queue)
    return context

# Fill in a pattern with variables in it using a list of variable bindings.
def construct(pattern, context):
    results = []
    for element in pattern:
        if element[0] == "VAR":
            if element[1] in context:
                for element in context[element[1]]:
                    results.append(element)
            else:
                results.append(element)
        else:
            results.append(element)
    return results

# Apply a pattern/replacement rule to the queue.
def apply(queue, rules, pattern, replacement):
    context = match(pattern, queue)
    if context == None:
        return (False, roll(queue))
    pattern = construct(pattern, context)
    if not pattern:
        return (False, roll(queue))
    replacement = construct(replacement, context)
    return (True, enqueue(dequeue(queue, len(pattern)), replacement))

def define(queue, rules, pattern):
    context = match(pattern, queue)
    left    = context["left"]
    right   = context["right"]
    if right and left:
        if len(left) > 1:
            left = left[1:][:-1]
        if len(right) > 1:
            right = right[1:][:-1]
        rules.append((left, apply, [right]))
        return (True, dequeue(queue, len(construct(pattern, context))))
    return (False, roll(queue))

def undefine(queue, rules, pattern):
    context = match(pattern, queue)
    left    = context["left"]
    if left:
        if len(left) > 1:
            left = left[1:][:-1]
        for rule, index in zip(rules, range(0, len(rules))):
            candidate , _, _ = rule
            if candidate == left:
                del rules[index]
        return (True, dequeue(queue, len(construct(pattern, context))))
    return (False, roll(queue))

def add(queue, rules, pattern):
    context = match(pattern, queue)
    left    = context["left"]
    right   = context["right"]
    if left and right:
        if left[0][0] == "NUM" and right[0][0] == "NUM":
            queue = dequeue(queue, len(construct(pattern, context)))
            queue = enqueue(queue, parse(str(left[0][1] + right[0][1])))
            return (True, queue)
    return (False, roll(queue))

def subtract(queue, rules, pattern):
    context = match(pattern, queue)
    left    = context["left"]
    right   = context["right"]
    if left and right:
        if left[0][0] == "NUM" and right[0][0] == "NUM":
            queue = dequeue(queue, len(construct(pattern, context)))
            queue = enqueue(queue, parse(str(left[0][1] - right[0][1])))
            return (True, queue)
    return (False, roll(queue))

def multiply(queue, rules, pattern):
    context = match(pattern, queue)
    left    = context["left"]
    right   = context["right"]
    if left and right:
        if left[0][0] == "NUM" and right[0][0] == "NUM":
            queue = dequeue(queue, len(construct(pattern, context)))
            queue = enqueue(queue, parse(str(left[0][1] * right[0][1])))
            return (True, queue)
    return (False, roll(queue))

def divide(queue, rules, pattern):
    context = match(pattern, queue)
    left    = context["left"]
    right   = context["right"]
    if left and right:
        if left[0][0] == "NUM" and right[0][0] == "NUM":
            queue = dequeue(queue, len(construct(pattern, context)))
            queue = enqueue(queue, parse(str(left[0][1] // right[0][1])))
            return (True, queue)
    return (False, roll(queue))

def modulo(queue, rules, pattern):
    context = match(pattern, queue)
    left    = context["left"]
    right   = context["right"]
    if left and right:
        if left[0][0] == "NUM" and right[0][0] == "NUM":
            queue = dequeue(queue, len(construct(pattern, context)))
            queue = enqueue(queue, parse(str(left[0][1] % right[0][1])))
            return (True, queue)
    return (False, roll(queue))

def display(queue, rules, pattern):
    context = match(pattern, queue)
    left    = context["left"]
    if left:
        if left[0][0] == "LIT" or left[0][0] == "NUM":
            if left[0][1] == "space":
                print(' ', end="")
            elif left[0][1] == "newline":
                print('\n', end="")
            elif left[0][1] == "tab":
                print('\t', end="")
            else:
                print(left[0][1], end="")
            return (True, dequeue(queue, len(construct(pattern, context))))
    return (False, roll(queue))

def applicable(rules, queue):
    results = []
    for pattern, operation, parameters in rules:
        if match(pattern, queue) != None:
            results.append((pattern, operation, parameters))
    return results

def pick(list):
    if len(list) == 0:
        return None
    return list[0]

def reconstruct(pattern):
    for element in pattern:
        if element[0] == "INC":
            yield '('
        elif element[0] == "DEC":
            yield ')'
        elif element[0] == "NUM":
            yield str(element[1])
        elif element[0] == "LIT":
            yield element[1]
        elif element[0] == "VAR":
            yield '?' + element[1]
        yield ' '

def number(string):
    try:
        result = int(string)
        return True
    except:
        return False

def literal(string, index=0):
    token = ""
    while index != len(string):
        character = string[index]
        if character in ['(', ')', '{', '}', '[', ']', '?', ' ', '\t', '\n', '\r']:
            break
        else:
            token = token + string[index]
        index = index + 1
    return (token, index)

def parse(string, index=0):
    results = []
    while index != len(string):
        character = string[index]
        if character in ['(', ')', '{', '}', '[', ']', '?', ' ', '\t', '\n', '\r']:
            index = index + 1
        if character in ['(', '{', '[']:
            results.append(["INC"])
        elif character in [')', '}', ']']:
            results.append(["DEC"])
        elif character not in [' ', '\t', '\n', '\r']:
            token, index = literal(string, index)
            if character == '?':
                results.append(["VAR", token])
            elif number(token):
                results.append(["NUM", int(token)])
            else:
                results.append(["LIT", token])
    return results

@measure
def run(rules, queue, limit=pow(2, 32)):
    steps      = 0
    failures   = 0
    queue      = [["SRT"]] + queue
    while failures != len(queue) and steps != limit:
        rule = pick(applicable(rules, queue))
        if rule == None:
            queue = roll(queue)
            failures = failures + 1
        else:
            pattern, operation, parameters = rule
            result, queue = operation(queue, rules, pattern, *parameters)
            if result == True:
                failures = 0
            #print("<>:   ", inspect(seek(queue, ["SRT"])))
            #print("<>:   ", inspect(queue))
            #input()
        steps = steps + 1
    if steps == limit:
        print("Execution limit reached.")
    return queue

def read(file):
    try:
        with open(file) as file:
            content = file.read()
        return content
    except EnvironmentError:
        return None

def inspect(pattern):
    return "".join(reconstruct(pattern))

def usage(name):
    print(name + ':', "a programming language based on rewriting.")
    print("Usage:")
    print((' ' * 4) + name, "<file>")

def prompt(prompt):
    try:
        return input(prompt)
    except:
        return None

def help():
    prefix = ' ' * 4
    print("Commands:")
    print(prefix + "rules : Display the current ruleset.")
    print(prefix + "clear : Clear the current ruleset.")
    print(prefix + "help  : This message.")
    print(prefix + "quit  : Quit.")

def main():
    defaults = [
                   (parse("define ?left ?right"),       define,   []),
                   (parse("undefine ?left"),            undefine, []),
                   (parse("add (?left) (?right)"),      add,      []),
                   (parse("add (?left) ?right"),        add,      []),
                   (parse("add ?left (?right)"),        add,      []),
                   (parse("add ?left ?right"),          add,      []),
                   (parse("subtract (?left) (?right)"), subtract, []),
                   (parse("subtract (?left) ?right"),   subtract, []),
                   (parse("subtract ?left (?right)"),   subtract, []),
                   (parse("subtract ?left ?right"),     subtract, []),
                   (parse("multiply (?left) (?right)"), multiply, []),
                   (parse("multiply (?left) ?right"),   multiply, []),
                   (parse("multiply ?left (?right)"),   multiply, []),
                   (parse("multiply ?left ?right"),     multiply, []),
                   (parse("divide   (?left) (?right)"), divide,   []),
                   (parse("divide   (?left) ?right"),   divide,   []),
                   (parse("divide   ?left (?right)"),   divide,   []),
                   (parse("divide   ?left ?right"),     divide,   []),
                   (parse("modulo   (?left) (?right)"), modulo,   []),
                   (parse("modulo   (?left) ?right"),   modulo,   []),
                   (parse("modulo   ?left (?right)"),   modulo,   []),
                   (parse("modulo   ?left ?right"),     modulo,   []),
                   (parse("(?left) + (?right)"),        add,      []),
                   (parse("(?left) + ?right"),          add,      []),
                   (parse("?left   + (?right)"),        add,      []),
                   (parse("?left   + ?right"),          add,      []),
                   (parse("(?left) - (?right)"),        subtract, []),
                   (parse("(?left) - ?right"),          subtract, []),
                   (parse("?left   - (?right)"),        subtract, []),
                   (parse("?left   - ?right"),          subtract, []),
                   (parse("(?left) * (?right)"),        multiply, []),
                   (parse("(?left) * ?right"),          multiply, []),
                   (parse("?left   * (?right)"),        multiply, []),
                   (parse("?left   * ?right"),          multiply, []),
                   (parse("?left / ?right"),            divide,   []),
                   (parse("?left % ?right"),            modulo,   []),
                   (parse("display ?left"),             display,  []),
               ]
    rules = defaults.copy()
    if len(sys.argv) >= 2:
        content = read(sys.argv[1])
        if content == None:
            print("No such file.")
            return
        print("Initializating...")
        run(rules, parse(content))
    else:
        usage(sys.argv[0])
        return
    print("Modal v0.02")
    help()
    while True:
        input = prompt("::> ")
        if input == None:
            break
        elif parse(input) == parse("rules"):
            print("Rules:")
            prefix = ' ' * 4
            for pattern, operation, parameters in rules:
                if operation == apply:
                    print(prefix + inspect(pattern) + "-> " + inspect(parameters[0]))
        elif parse(input) == parse("clear"):
            rules = defaults
        elif parse(input) == parse("help"):
            help()
        elif parse(input) == parse("quit") or parse(input) == parse("exit"):
            break
        else:
            print("Reducing...")
            print(inspect(seek(run(rules, parse(input)), ["SRT"])))
    return

if __name__ == "__main__":
    main()