# Source code for joy.library

    # -*- coding: utf-8 -*-
    #
    #    Copyright © 2014-2020 Simon Forman
    #
    #    This file is part of Thun
    #
    #    Thun is free software: you can redistribute it and/or modify
    #    it under the terms of the GNU General Public License as published by
    #    the Free Software Foundation, either version 3 of the License, or
    #    (at your option) any later version.
    #
    #    Thun is distributed in the hope that it will be useful,
    #    but WITHOUT ANY WARRANTY; without even the implied warranty of
    #    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    #    GNU General Public License for more details.
    #
    #    You should have received a copy of the GNU General Public License
    #    along with Thun.  If not see <http://www.gnu.org/licenses/>.
    #
    '''
    This module contains the Joy function infrastructure and a library of
    functions.  Its main export is a Python function initialize() that
    returns a dictionary of Joy functions suitable for use with the joy()
    function.
    '''
    from pkg_resources import resource_stream
    from io import TextIOWrapper
    from inspect import getdoc, getmembers, isfunction
    from functools import wraps
    from itertools import count
    import operator, math

    from . import __name__ as _joy_package_name
    from .parser import text_to_expression, Symbol
    from .utils import generated_library as genlib
    from .utils.errors import (
        NotAListError,
        NotAnIntError,
        StackUnderflowError,
        )
    from .utils.stack import (
        concat,
        expression_to_string,
        iter_stack,
        list_to_stack,
        pick,
        )

    def default_defs(dictionary):
        def_stream = TextIOWrapper(
            resource_stream(_joy_package_name, 'defs.txt'),
            encoding='UTF_8',
            )
        Def.load_definitions(def_stream, dictionary)

    HELP_TEMPLATE = '''\

    ==== Help on %s ====

    %s

    ---- end (%s)
    '''

    # This is the main dict we're building.
    _dictionary = {}

    def inscribe(function, d=_dictionary):
        '''A decorator to inscribe functions into the default dictionary.'''
        d[function.name] = function
        return function

    def initialize():
        '''Return a dictionary of Joy functions for use with joy().'''
        return _dictionary.copy()

    ALIASES = (
        ('add', ['+']),
        ('and', ['&']),
        ('bool', ['truthy']),
        ('mul', ['*']),
        ('floordiv', ['/floor', '//', '/', 'div']),
        ('mod', ['%', 'rem', 'remainder', 'modulus']),
        ('eq', ['=']),
        ('ge', ['>=']),
        ('getitem', ['pick', 'at']),
        ('gt', ['>']),
        ('le', ['<=']),
        ('lshift', ['<<']),
        ('lt', ['<']),
        ('ne', ['<>', '!=']),
        ('rshift', ['>>']),
        ('sub', ['-']),
        ('xor', ['^']),
        ('succ', ['++']),
        ('pred', ['--']),
        ('rolldown', ['roll<']),
        ('rollup', ['roll>']),
        ('eh', ['?']),
        ('id', [u'•']),
        )

    def add_aliases(D, A):
        '''
        Given a dict and a iterable of (name, [alias, ...]) pairs, create
        additional entries in the dict mapping each alias to the named function
        if it's in the dict.  Aliases for functions not in the dict are ignored.
        '''
        for name, aliases in A:
            try:
                F = D[name]
            except KeyError:
                continue
            for alias in aliases:
                D[alias] = F

    def FunctionWrapper(f):
        '''Set name attribute.'''
        if not f.__doc__:
            raise ValueError('Function %s must have doc string.' % f.__name__)
        f.name = f.__name__.rstrip('_')  # Don't shadow builtins.
        return f

    def SimpleFunctionWrapper(f):
        '''
        Wrap functions that take and return just a stack.
        '''
        @FunctionWrapper
        @wraps(f)
        def inner(stack, expression, dictionary):
            return f(stack), expression, dictionary
        return inner

    def BinaryBuiltinWrapper(f):
        '''
        Wrap functions that take two arguments and return a single result.
        '''
        @FunctionWrapper
        @wraps(f)
        def inner(stack, expression, dictionary):
            try:
                (a, (b, stack)) = stack
            except ValueError:
                raise StackUnderflowError('Not enough values on stack.')
            # Boolean predicates like "or" fail here.  :(
    ##        if (   not isinstance(a, int)
    ##            or not isinstance(b, int)
    ##            or isinstance(a, bool)  # Because bools are ints in Python.
    ##            or isinstance(b, bool)
    ##            ):
    ##            raise NotAnIntError
            result = f(b, a)
            return (result, stack), expression, dictionary
        return inner

    def UnaryBuiltinWrapper(f):
        '''
        Wrap functions that take one argument and return a single result.
        '''
        @FunctionWrapper
        @wraps(f)
        def inner(stack, expression, dictionary):
            (a, stack) = stack
            result = f(a)
            return (result, stack), expression, dictionary
        return inner

    class Def(object):
        '''
        Definitions created by inscribe.
        '''

        def __init__(self, name, body):
            self.name = name
            self.body = body
            self._body = tuple(iter_stack(body))
            self.__doc__ = expression_to_string(body)
            self._compiled = None

        def __call__(self, stack, expression, dictionary):
            if self._compiled:
                return self._compiled(stack, expression, dictionary)  # pylint: disable=E1102
            expression = list_to_stack(self._body, expression)
            return stack, expression, dictionary

        @classmethod
        def load_definitions(class_, stream, dictionary):
            for line in stream:
                if line.lstrip().startswith('#'):
                    continue
                name, body = text_to_expression(line)
                if name not in dictionary:
                    inscribe(class_(name, body), dictionary)
    ##            inscribe(class_(name, body), dictionary)

    #
    # Functions
    #

    @inscribe
    @FunctionWrapper
    def inscribe_(stack, expression, dictionary):
        '''
        Create a new Joy function definition in the Joy dictionary.  A
        definition is given as a quote with a name followed by a Joy
        expression. for example:

            [sqr dup mul] inscribe

        '''
        (name, body), stack = stack
        inscribe(Def(name, body), dictionary)
        return stack, expression, dictionary

    @inscribe
    @SimpleFunctionWrapper
    def parse(stack):
        '''Parse the string on the stack to a Joy expression.'''
        text, stack = stack
        expression = text_to_expression(text)
        return expression, stack

    # @inscribe
    # @SimpleFunctionWrapper
    # def infer_(stack):
    #     '''Attempt to infer the stack effect of a Joy expression.'''
    #     E, stack = stack
    #     effects = infer_expression(E)
    #     e = list_to_stack([(fi, (fo, ())) for fi, fo in effects])
    #     return e, stack

    @inscribe
    @SimpleFunctionWrapper
    def getitem(stack):
        '''
        ::

            getitem == drop first

        Expects an integer and a quote on the stack and returns the item at the
        nth position in the quote counting from 0.
        ::

               [a b c d] 0 getitem
            -------------------------
                a

        '''
        n, (Q, stack) = stack
        return pick(Q, n), stack

    @inscribe
    @SimpleFunctionWrapper
    def drop(stack):
        '''
        ::

            drop == [rest] times

        Expects an integer and a quote on the stack and returns the quote with
        n items removed off the top.
        ::

               [a b c d] 2 drop
            ----------------------
                   [c d]

        '''
        n, (Q, stack) = stack
        while n > 0:
            try:
                _, Q = Q
            except ValueError:
                raise IndexError
            n -= 1
        return Q, stack

    @inscribe
    @SimpleFunctionWrapper
    def take(stack):
        '''
        Expects an integer and a quote on the stack and returns the quote with
        just the top n items in reverse order (because that's easier and you can
        use reverse if needed.)
        ::

               [a b c d] 2 take
            ----------------------
                   [b a]

        '''
        n, (Q, stack) = stack
        x = ()
        while n > 0:
            try:
                item, Q = Q
            except ValueError:
                raise IndexError
            x = item, x
            n -= 1
        return x, stack

    @inscribe
    @FunctionWrapper
    def gcd2(stack, expression, dictionary):
        '''Compiled GCD function.'''
        (v1, (v2, stack)) = stack
        tos = True
        while tos:
            v3 = v2 % v1
            tos = v3 > 0
            (v1, (v2, stack)) = (v3, (v1, stack))
        return (v2, stack), expression, dictionary

    @inscribe
    @SimpleFunctionWrapper
    def choice(stack):
        '''
        Use a Boolean value to select one of two items.
        ::

               A B False choice
            ----------------------
               A

               A B True choice
            ---------------------
                 B

        Currently Python semantics are used to evaluate the "truthiness" of the
        Boolean value (so empty string, zero, etc. are counted as false, etc.)
        '''
        (if_, (then, (else_, stack))) = stack
        return then if if_ else else_, stack

    @inscribe
    @SimpleFunctionWrapper
    def select(stack):
        '''
        Use a Boolean value to select one of two items from a sequence.
        ::

               [A B] False select
            ------------------------
                A

               [A B] True select
            -----------------------
                  B

        The sequence can contain more than two items but not fewer.
        Currently Python semantics are used to evaluate the "truthiness" of the
        Boolean value (so empty string, zero, etc. are counted as false, etc.)
        '''
        (flag, (choices, stack)) = stack
        (else_, (then, _)) = choices
        return then if flag else else_, stack

    @inscribe
    @SimpleFunctionWrapper
    def max_(S):
        '''Given a list find the maximum.'''
        tos, stack = S
        return max(iter_stack(tos)), stack

    @inscribe
    @SimpleFunctionWrapper
    def min_(S):
        '''Given a list find the minimum.'''
        tos, stack = S
        return min(iter_stack(tos)), stack

    @inscribe
    @SimpleFunctionWrapper
    def sum_(S):
        '''
        Given a quoted sequence of numbers return the sum.
        ::

            sum == 0 swap [+] step

        '''
        tos, stack = S
        return sum(iter_stack(tos)), stack

    @inscribe
    @SimpleFunctionWrapper
    def remove(S):
        '''
        Expects an item on the stack and a quote under it and removes that item
        from the the quote.  The item is only removed once.  If the list is
        empty or the item isn't in the list then the list is unchanged.
        ::

               [1 2 3 1] 1 remove
            ------------------------
                 [2 3 1]

        '''
        (item, (quote, stack)) = S
        return _remove(item, quote), stack

    def _remove(item, quote):
        try: head, tail = quote
        except ValueError: return quote
        return tail if head == item else (head, _remove(item, tail))

    @inscribe
    @SimpleFunctionWrapper
    def unique(S):
        '''Given a list remove duplicate items.'''
        tos, stack = S
        I = list(iter_stack(tos))
        return list_to_stack(sorted(set(I), key=I.index)), stack

    @inscribe
    @SimpleFunctionWrapper
    def sort_(S):
        '''Given a list return it sorted.'''
        tos, stack = S
        return list_to_stack(sorted(iter_stack(tos))), stack

    @inscribe
    @SimpleFunctionWrapper
    def clear(stack):
        '''Clear everything from the stack.
        ::

            clear == stack [pop stack] loop

               ... clear
            ---------------

        '''
        return ()

    @inscribe
    @SimpleFunctionWrapper
    def disenstacken(stack):
        '''
        The disenstacken operator expects a list on top of the stack and makes that
        the stack discarding the rest of the stack.
        '''
        return stack[0]

    @inscribe
    @SimpleFunctionWrapper
    def reverse(S):
        '''
        Reverse the list on the top of the stack.
        ::

            reverse == [] swap shunt
        '''
        (tos, stack) = S
        res = ()
        for term in iter_stack(tos):
            res = term, res
        return res, stack

    @inscribe
    @SimpleFunctionWrapper
    def concat_(S):
        '''
        Concatinate the two lists on the top of the stack.
        ::

               [a b c] [d e f] concat
            ----------------------------
                   [a b c d e f]

        '''
        (tos, (second, stack)) = S
        return concat(second, tos), stack

    @inscribe
    @SimpleFunctionWrapper
    def shunt(stack):
        '''
        Like concat but reverses the top list into the second.
        ::

            shunt == [swons] step == reverse swap concat

               [a b c] [d e f] shunt
            ---------------------------
                   [f e d a b c]

        '''
        (tos, (second, stack)) = stack
        while tos:
            term, tos = tos
            second = term, second
        return second, stack

    @inscribe
    @SimpleFunctionWrapper
    def zip_(S):
        '''
        Replace the two lists on the top of the stack with a list of the pairs
        from each list.  The smallest list sets the length of the result list.
        '''
        (tos, (second, stack)) = S
        accumulator = [
            (a, (b, ()))
            for a, b in zip(iter_stack(tos), iter_stack(second))
            ]
        return list_to_stack(accumulator), stack

    @inscribe
    @SimpleFunctionWrapper
    def succ(S):
        '''Increment TOS.'''
        (tos, stack) = S
        return tos + 1, stack

    @inscribe
    @SimpleFunctionWrapper
    def pred(S):
        '''Decrement TOS.'''
        (tos, stack) = S
        return tos - 1, stack

    @inscribe
    @SimpleFunctionWrapper
    def pm(stack):
        '''
        Plus or minus
        ::

               a b pm
            -------------
               a+b a-b

        '''
        a, (b, stack) = stack
        p, m, = b + a, b - a
        return m, (p, stack)

    def floor(n):
        return int(math.floor(n))

    floor.__doc__ = math.floor.__doc__

    @inscribe
    @SimpleFunctionWrapper
    def divmod_(S):
        '''
        divmod(x, y) -> (quotient, remainder)

        Return the tuple (x//y, x%y).  Invariant: q * y + r == x.
        '''
        a, (b, stack) = S
        d, m = divmod(a, b)
        return d, (m, stack)

    def sqrt(a):
        '''
        Return the square root of the number a.
        Negative numbers return complex roots.
        '''
        try:
            r = math.sqrt(a)
        except ValueError:
            assert a < 0, repr(a)
            r = math.sqrt(-a) * 1j
        return r

    #def execute(S):
    #  (text, stack) = S
    #  if isinstance(text, str):
    #    return run(text, stack)
    #  return stack

    @inscribe
    @SimpleFunctionWrapper
    def id_(stack):
        '''The identity function.'''
        return stack

    @inscribe
    @SimpleFunctionWrapper
    def void(stack):
        '''True if the form on TOS is void otherwise False.'''
        form, stack = stack
        return _void(form), stack

    def _void(form):
        return any(not _void(i) for i in iter_stack(form))

    ##  transpose
    ##  sign
    ##  take

    @inscribe
    @FunctionWrapper
    def words(stack, expression, dictionary):
        '''Print all the words in alphabetical order.'''
        print(' '.join(sorted(dictionary)))
        return stack, expression, dictionary

    @inscribe
    @FunctionWrapper
    def sharing(stack, expression, dictionary):
        '''Print redistribution information.'''
        print("You may convey verbatim copies of the Program's source code as"
        ' you receive it, in any medium, provided that you conspicuously'
        ' and appropriately publish on each copy an appropriate copyright'
        ' notice; keep intact all notices stating that this License and'
        ' any non-permissive terms added in accord with section 7 apply'
        ' to the code; keep intact all notices of the absence of any'
        ' warranty; and give all recipients a copy of this License along'
        ' with the Program.'
        ' You should have received a copy of the GNU General Public License'
        ' along with Thun.  If not see <http://www.gnu.org/licenses/>.')
        return stack, expression, dictionary

    @inscribe
    @FunctionWrapper
    def warranty(stack, expression, dictionary):
        '''Print warranty information.'''
        print('THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY'
        ' APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE'
        ' COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM'
        ' "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR'
        ' IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES'
        ' OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE'
        ' ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS'
        ' WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE'
        ' COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.')
        return stack, expression, dictionary

    # def simple_manual(stack):
    #   '''
    #   Print words and help for each word.
    #   '''
    #   for name, f in sorted(FUNCTIONS.items()):
    #     d = getdoc(f)
    #     boxline = '+%s+' % ('-' * (len(name) + 2))
    #     print('\n'.join((
    #       boxline,
    #       '| %s |' % (name,),
    #       boxline,
    #       d if d else '   ...',
    #       '',
    #       '--' * 40,
    #       '',
    #       )))
    #   return stack

    @inscribe
    @FunctionWrapper
    def help_(S, expression, dictionary):
        '''Accepts a quoted symbol on the top of the stack and prints its docs.'''
        ((symbol, _), stack) = S
        word = dictionary[symbol]
        print(HELP_TEMPLATE % (symbol, getdoc(word), symbol))
        return stack, expression, dictionary

    #
    # § Combinators
    #

    # Several combinators depend on other words in their definitions,
    # we use symbols to prevent hard-coding these, so in theory, you
    # could change the word in the dictionary to use different semantics.
    S_choice = Symbol('choice')
    S_first = Symbol('first')
    S_genrec = Symbol('genrec')
    S_getitem = Symbol('getitem')
    S_i = Symbol('i')
    S_ifte = Symbol('ifte')
    S_infra = Symbol('infra')
    S_loop = Symbol('loop')
    S_pop = Symbol('pop')
    S_primrec = Symbol('primrec')
    S_step = Symbol('step')
    S_swaack = Symbol('swaack')
    S_times = Symbol('times')

    @inscribe
    @FunctionWrapper
    def i(stack, expression, dictionary):
        '''
        The i combinator expects a quoted program on the stack and unpacks it
        onto the pending expression for evaluation.
        ::

               [Q] i
            -----------
                Q

        '''
        try:
            quote, stack = stack
        except ValueError:
            raise StackUnderflowError('Not enough values on stack.')
        return stack, concat(quote, expression), dictionary

    @inscribe
    @FunctionWrapper
    def x(stack, expression, dictionary):
        '''
        ::

            x == dup i

            ... [Q] x = ... [Q] dup i
            ... [Q] x = ... [Q] [Q] i
            ... [Q] x = ... [Q]  Q

        '''
        quote, _ = stack
        return stack, concat(quote, expression), dictionary

    @inscribe
    @FunctionWrapper
    def b(stack, expression, dictionary):
        '''
        ::

            b == [i] dip i

            ... [P] [Q] b == ... [P] i [Q] i
            ... [P] [Q] b == ... P Q

        '''
        q, (p, (stack)) = stack
        return stack, concat(p, concat(q, expression)), dictionary

    @inscribe
    @FunctionWrapper
    def ii(stack, expression, dictionary):
        '''
        ::

               ... a [Q] ii
            ------------------
                ... Q a Q

        '''
        quote, (a, stack) = stack
        expression = concat(quote, (a, concat(quote, expression)))
        return stack, expression, dictionary

    @inscribe
    @FunctionWrapper
    def dupdip(stack, expression, dictionary):
        '''
        ::

            [F] dupdip == dup [F] dip

            ... a [F] dupdip
            ... a dup [F] dip
            ... a a   [F] dip
            ... a F a

        '''
        F, stack = stack
        a = stack[0]
        return stack, concat(F, (a,  expression)), dictionary

    @inscribe
    @FunctionWrapper
    def infra(stack, expression, dictionary):
        '''
        Accept a quoted program and a list on the stack and run the program
        with the list as its stack.  Does not affect the rest of the stack.
        ::

               ... [a b c] [Q] . infra
            -----------------------------
                c b a . Q [...] swaack

        '''
        (quote, (aggregate, stack)) = stack
        return aggregate, concat(quote, (stack, (S_swaack, expression))), dictionary

    @inscribe
    @FunctionWrapper
    def genrec(stack, expression, dictionary):
        '''
        General Recursion Combinator.
        ::

                      [if] [then] [rec1] [rec2] genrec
            ---------------------------------------------------------------------
               [if] [then] [rec1 [[if] [then] [rec1] [rec2] genrec] rec2] ifte

        From "Recursion Theory and Joy" (j05cmp.html) by Manfred von Thun:
        "The genrec combinator takes four program parameters in addition to
        whatever data parameters it needs. Fourth from the top is an if-part,
        followed by a then-part. If the if-part yields true, then the then-part
        is executed and the combinator terminates. The other two parameters are
        the rec1-part and the rec2-part. If the if-part yields false, the
        rec1-part is executed. Following that the four program parameters and
        the combinator are again pushed onto the stack bundled up in a quoted
        form. Then the rec2-part is executed, where it will find the bundled
        form. Typically it will then execute the bundled form, either with i or
        with app2, or some other combinator."

        The way to design one of these is to fix your base case [then] and the
        test [if], and then treat rec1 and rec2 as an else-part "sandwiching"
        a quotation of the whole function.

        For example, given a (general recursive) function 'F':
        ::

            F == [I] [T] [R1] [R2] genrec

        If the [I] if-part fails you must derive R1 and R2 from:
        ::

            ... R1 [F] R2

        Just set the stack arguments in front, and figure out what R1 and R2
        have to do to apply the quoted [F] in the proper way.  In effect, the
        genrec combinator turns into an ifte combinator with a quoted copy of
        the original definition in the else-part:
        ::

            F == [I] [T] [R1]   [R2] genrec
              == [I] [T] [R1 [F] R2] ifte

        Primitive recursive functions are those where R2 == i.
        ::

            P == [I] [T] [R] tailrec
              == [I] [T] [R [P] i] ifte
              == [I] [T] [R P] ifte

        '''
        (rec2, (rec1, stack)) = stack
        (then, (if_, _)) = stack
        F = (if_, (then, (rec1, (rec2, (S_genrec, ())))))
        else_ = concat(rec1, (F, rec2))
        return (else_, stack), (S_ifte, expression), dictionary

    @inscribe
    @FunctionWrapper
    def map_(S, expression, dictionary):
        '''
        Run the quoted program on TOS on the items in the list under it, push a
        new list with the results in place of the program and original list.
        '''
        # (quote, (aggregate, stack)) = S
        # results = list_to_stack([
        # joy((term, stack), quote, dictionary)[0][0]
        # for term in iter_stack(aggregate)
        # ])
        # return (results, stack), expression, dictionary
        (quote, (aggregate, stack)) = S
        if not aggregate:
            return (aggregate, stack), expression, dictionary
        batch = ()
        for term in iter_stack(aggregate):
            s = term, stack
            batch = (s, (quote, (S_infra, (S_first, batch))))
        stack = (batch, ((), stack))
        return stack, (S_infra, expression), dictionary

    @inscribe
    @FunctionWrapper
    def primrec(stack, expression, dictionary):
        '''
        From the "Overview of the language JOY":

        > The primrec combinator expects two quoted programs in addition to a
        data parameter. For an integer data parameter it works like this: If
        the data parameter is zero, then the first quotation has to produce
        the value to be returned. If the data parameter is positive then the
        second has to combine the data parameter with the result of applying
        the function to its predecessor.::

            5  [1]  [*]  primrec

        > Then primrec tests whether the top element on the stack (initially
        the 5) is equal to zero. If it is, it pops it off and executes one of
        the quotations, the [1] which leaves 1 on the stack as the result.
        Otherwise it pushes a decremented copy of the top element and
        recurses. On the way back from the recursion it uses the other
        quotation, [*], to multiply what is now a factorial on top of the
        stack by the second element on the stack.::

            n [Base] [Recur] primrec

               0 [Base] [Recur] primrec
            ------------------------------
                  Base

                 n [Base] [Recur] primrec
            ------------------------------------------ n > 0
               n (n-1) [Base] [Recur] primrec Recur

        '''
        recur, (base, (n, stack)) = stack
        if n <= 0:
            expression = concat(base, expression)
        else:
            expression = S_primrec, concat(recur, expression)
            stack = recur, (base, (n - 1, (n, stack)))
        return stack, expression, dictionary

    #def cleave(S, expression, dictionary):
    #  '''
    #  The cleave combinator expects two quotations, and below that an item X.
    #  It first executes [P], with X on top, and saves the top result element.
    #  Then it executes [Q], again with X, and saves the top result.
    #  Finally it restores the stack to what it was below X and pushes the two
    #  results P(X) and Q(X).
    #  '''
    #  (Q, (P, (x, stack))) = S
    #  p = joy((x, stack), P, dictionary)[0][0]
    #  q = joy((x, stack), Q, dictionary)[0][0]
    #  return (q, (p, stack)), expression, dictionary

    @inscribe
    @FunctionWrapper
    def branch(stack, expression, dictionary):
        '''
        Use a Boolean value to select one of two quoted programs to run.

        ::

            branch == roll< choice i

        ::

               False [F] [T] branch
            --------------------------
                  F

               True [F] [T] branch
            -------------------------
                     T

        '''
        (then, (else_, (flag, stack))) = stack
        return stack, concat(then if flag else else_, expression), dictionary

    @inscribe
    @FunctionWrapper
    def ifte(stack, expression, dictionary):
      '''
      If-Then-Else Combinator
      ::

                      ... [if] [then] [else] ifte
           ---------------------------------------------------
              ... [[else] [then]] [...] [if] infra select i

                    ... [if] [then] [else] ifte
           -------------------------------------------------------
              ... [else] [then] [...] [if] infra first choice i

      Has the effect of grabbing a copy of the stack on which to run the
      if-part using infra.
      '''
      (else_, (then, (if_, stack))) = stack
      expression = (S_infra, (S_first, (S_choice, (S_i, expression))))
      stack = (if_, (stack, (then, (else_, stack))))
      return stack, expression, dictionary

    @inscribe
    @FunctionWrapper
    def cond(stack, expression, dictionary):
        '''
        This combinator works like a case statement.  It expects a single quote
        on the stack that must contain zero or more condition quotes and a
        default quote.  Each condition clause should contain a quoted predicate
        followed by the function expression to run if that predicate returns
        true.  If no predicates return true the default function runs.

        It works by rewriting into a chain of nested `ifte` expressions, e.g.::

                  [[[B0] T0] [[B1] T1] [D]] cond
            -----------------------------------------
               [B0] [T0] [[B1] [T1] [D] ifte] ifte

        '''
        conditions, stack = stack
        if conditions:
            expression = _cond(conditions, expression)
            try:
                # Attempt to preload the args to first ifte.
                (P, (T, (E, expression))) = expression
            except ValueError:
                # If, for any reason, the argument to cond should happen to contain
                # only the default clause then this optimization will fail.
                pass
            else:
                stack = (E, (T, (P, stack)))
        return stack, expression, dictionary

    def _cond(conditions, expression):
        (clause, rest) = conditions
        if not rest:  # clause is [D]
            return clause
        P, T = clause
        return (P, (T, (_cond(rest, ()), (S_ifte, expression))))

    @inscribe
    @FunctionWrapper
    def dip(stack, expression, dictionary):
        '''
        The dip combinator expects a quoted program on the stack and below it
        some item, it hoists the item into the expression and runs the program
        on the rest of the stack.
        ::

               ... x [Q] dip
            -------------------
                 ... Q x

        '''
        try:
            (quote, (x, stack)) = stack
        except ValueError:
            raise StackUnderflowError('Not enough values on stack.')
        expression = (x, expression)
        return stack, concat(quote, expression), dictionary

    @inscribe
    @FunctionWrapper
    def dipd(S, expression, dictionary):
        '''
        Like dip but expects two items.
        ::

               ... y x [Q] dip
            ---------------------
                 ... Q y x

        '''
        (quote, (x, (y, stack))) = S
        expression = (y, (x, expression))
        return stack, concat(quote, expression), dictionary

    @inscribe
    @FunctionWrapper
    def dipdd(S, expression, dictionary):
        '''
        Like dip but expects three items.
        ::

               ... z y x [Q] dip
            -----------------------
                 ... Q z y x

        '''
        (quote, (x, (y, (z, stack)))) = S
        expression = (z, (y, (x, expression)))
        return stack, concat(quote, expression), dictionary

    @inscribe
    @FunctionWrapper
    def app1(S, expression, dictionary):
        '''
        Given a quoted program on TOS and anything as the second stack item run
        the program and replace the two args with the first result of the
        program.
        ::

                 ... x [Q] . app1
            -----------------------------------
               ... [x ...] [Q] . infra first

        '''
        (quote, (x, stack)) = S
        stack = (quote, ((x, stack), stack))
        expression = (S_infra, (S_first, expression))
        return stack, expression, dictionary

    @inscribe
    @FunctionWrapper
    def app2(S, expression, dictionary):
        '''Like app1 with two items.
        ::

                   ... y x [Q] . app2
            -----------------------------------
               ... [y ...] [Q] . infra first
                   [x ...] [Q]   infra first

        '''
        (quote, (x, (y, stack))) = S
        expression = (S_infra, (S_first,
            ((x, stack), (quote, (S_infra, (S_first,
                expression))))))
        stack = (quote, ((y, stack), stack))
        return stack, expression, dictionary

    @inscribe
    @FunctionWrapper
    def app3(S, expression, dictionary):
        '''Like app1 with three items.
        ::

                 ... z y x [Q] . app3
            -----------------------------------
               ... [z ...] [Q] . infra first
                   [y ...] [Q]   infra first
                   [x ...] [Q]   infra first

        '''
        (quote, (x, (y, (z, stack)))) = S
        expression = (S_infra, (S_first,
            ((y, stack), (quote, (S_infra, (S_first,
            ((x, stack), (quote, (S_infra, (S_first,
                expression))))))))))
        stack = (quote, ((z, stack), stack))
        return stack, expression, dictionary

    @inscribe
    @FunctionWrapper
    def step(S, expression, dictionary):
        '''
        Run a quoted program on each item in a sequence.
        ::

               ... [] [Q] . step
            -----------------------
                  ... .

               ... [a] [Q] . step
            ------------------------
                 ... a . Q

               ... [a b c] [Q] . step
            ----------------------------------------
                     ... a . Q [b c] [Q] step

        The step combinator executes the quotation on each member of the list
        on top of the stack.
        '''
        (quote, (aggregate, stack)) = S
        if not aggregate:
            return stack, expression, dictionary
        head, tail = aggregate
        stack = quote, (head, stack)
        if tail:
            expression = tail, (quote, (S_step, expression))
        expression = S_i, expression
        return stack, expression, dictionary

    @inscribe
    @FunctionWrapper
    def times(stack, expression, dictionary):
        '''
        times == [-- dip] cons [swap] infra [0 >] swap while pop
        ::

               ... n [Q] . times
            ---------------------  w/ n <= 0
                 ... .

               ... 1 [Q] . times
            -----------------------
                 ... . Q

               ... n [Q] . times
            -------------------------------------  w/ n > 1
                 ... . Q (n - 1) [Q] times

        '''
        # times == [-- dip] cons [swap] infra [0 >] swap while pop
        (quote, (n, stack)) = stack
        if n <= 0:
            return stack, expression, dictionary
        n -= 1
        if n:
            expression = n, (quote, (S_times, expression))
        expression = concat(quote, expression)
        return stack, expression, dictionary

    # The current definition above works like this:

    #             [P] [Q] while
    # --------------------------------------
    #    [P] nullary [Q [P] nullary] loop

    #   while == [pop i not] [popop] [dudipd] tailrec

    #def while_(S, expression, dictionary):
    #  '''[if] [body] while'''
    #  (body, (if_, stack)) = S
    #  while joy(stack, if_, dictionary)[0][0]:
    #    stack = joy(stack, body, dictionary)[0]
    #  return stack, expression, dictionary

    @inscribe
    @FunctionWrapper
    def loop(stack, expression, dictionary):
        '''
        Basic loop combinator.
        ::

               ... True [Q] loop
            -----------------------
                  ... Q [Q] loop

               ... False [Q] loop
            ------------------------
                  ...

        '''
        try:
            quote, stack = stack
        except ValueError:
            raise StackUnderflowError('Not enough values on stack.')
        if not isinstance(quote, tuple):
            raise NotAListError('Loop body not a list.')
        try:
            (flag, stack) = stack
        except ValueError:
            raise StackUnderflowError('Not enough values on stack.')
        if flag:
            expression = concat(quote, (quote, (S_loop, expression)))
        return stack, expression, dictionary

    @inscribe
    @FunctionWrapper
    def cmp_(stack, expression, dictionary):
        '''
        cmp takes two values and three quoted programs on the stack and runs
        one of the three depending on the results of comparing the two values:
        ::

               a b [G] [E] [L] cmp
            ------------------------- a > b
                G

               a b [G] [E] [L] cmp
            ------------------------- a = b
                    E

               a b [G] [E] [L] cmp
            ------------------------- a < b
                    L
        '''
        L, (E, (G, (b, (a, stack)))) = stack
        expression = concat(G if a > b else L if a < b else E, expression)
        return stack, expression, dictionary

    #  FunctionWrapper(cleave),
    #  FunctionWrapper(while_),

    for F in (

        #divmod_ = pm = __(n2, n1), __(n4, n3)

        BinaryBuiltinWrapper(operator.eq),
        BinaryBuiltinWrapper(operator.ge),
        BinaryBuiltinWrapper(operator.gt),
        BinaryBuiltinWrapper(operator.le),
        BinaryBuiltinWrapper(operator.lt),
        BinaryBuiltinWrapper(operator.ne),

        BinaryBuiltinWrapper(operator.xor),
        BinaryBuiltinWrapper(operator.lshift),
        BinaryBuiltinWrapper(operator.rshift),

        BinaryBuiltinWrapper(operator.and_),
        BinaryBuiltinWrapper(operator.or_),

        BinaryBuiltinWrapper(operator.add),
        BinaryBuiltinWrapper(operator.floordiv),
        BinaryBuiltinWrapper(operator.mod),
        BinaryBuiltinWrapper(operator.mul),
        BinaryBuiltinWrapper(operator.pow),
        BinaryBuiltinWrapper(operator.sub),
    ##    BinaryBuiltinWrapper(operator.truediv),

        UnaryBuiltinWrapper(bool),
        UnaryBuiltinWrapper(operator.not_),

        UnaryBuiltinWrapper(abs),
        UnaryBuiltinWrapper(operator.neg),
        UnaryBuiltinWrapper(sqrt),

        UnaryBuiltinWrapper(floor),
        UnaryBuiltinWrapper(round),
        ):
        inscribe(F)
    del F  # Otherwise Sphinx autodoc will pick it up.

    for name, primitive in getmembers(genlib, isfunction):
        inscribe(SimpleFunctionWrapper(primitive))

    add_aliases(_dictionary, ALIASES)