# -*- coding: utf-8 -*-
r"""
Fraction Field of Integral Domains

AUTHORS:

- William Stein (with input from David Joyner, David Kohel, and Joe
  Wetherell)

- Burcin Erocal

- Julian Rüth (2017-06-27): embedding into the field of fractions and its
  section

EXAMPLES:

Quotienting is a constructor for an element of the fraction field::

    sage: R.<x> = QQ[]
    sage: (x^2-1)/(x+1)
    x - 1
    sage: parent((x^2-1)/(x+1))
    Fraction Field of Univariate Polynomial Ring in x over Rational Field

The GCD is not taken (since it doesn't converge sometimes) in the
inexact case::

    sage: Z.<z> = CC[]
    sage: I = CC.gen()
    sage: (1+I+z)/(z+0.1*I)
    (z + 1.00000000000000 + I)/(z + 0.100000000000000*I)
    sage: (1+I*z)/(z+1.1)
    (I*z + 1.00000000000000)/(z + 1.10000000000000)

TESTS::

    sage: F = FractionField(IntegerRing())
    sage: F == loads(dumps(F))
    True

::

    sage: F = FractionField(PolynomialRing(RationalField(),'x'))
    sage: F == loads(dumps(F))
    True

::

    sage: F = FractionField(PolynomialRing(IntegerRing(),'x'))
    sage: F == loads(dumps(F))
    True

::

    sage: F = FractionField(PolynomialRing(RationalField(),2,'x'))
    sage: F == loads(dumps(F))
    True
"""
# ****************************************************************************
#
#   Sage: System for Algebra and Geometry Experimentation
#
#       Copyright (C) 2005 William Stein <wstein@gmail.com>
#                     2017 Julian Rüth <julian.rueth@fsfe.org>
#
# This program 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 2 of the License, or
# (at your option) any later version.
#                  http://www.gnu.org/licenses/
# ****************************************************************************
from __future__ import absolute_import
from six.moves import range
import six

from . import ring
from . import fraction_field_element
import sage.misc.latex as latex
from sage.misc.cachefunc import cached_method

from sage.rings.integer_ring import ZZ
from sage.structure.element import Element
from sage.structure.richcmp import richcmp
from sage.structure.parent import Parent
from sage.structure.coerce import py_scalar_to_element
from sage.structure.coerce_maps import CallableConvertMap, DefaultConvertMap_unique
from sage.categories.basic import QuotientFields
from sage.categories.morphism import Morphism
from sage.categories.map import Section


def FractionField(R, names=None):
    """
    Create the fraction field of the integral domain ``R``.

    INPUT:

    -  ``R`` -- an integral domain

    -  ``names`` -- ignored

    EXAMPLES:

    We create some example fraction fields::

        sage: FractionField(IntegerRing())
        Rational Field
        sage: FractionField(PolynomialRing(RationalField(),'x'))
        Fraction Field of Univariate Polynomial Ring in x over Rational Field
        sage: FractionField(PolynomialRing(IntegerRing(),'x'))
        Fraction Field of Univariate Polynomial Ring in x over Integer Ring
        sage: FractionField(PolynomialRing(RationalField(),2,'x'))
        Fraction Field of Multivariate Polynomial Ring in x0, x1 over Rational Field

    Dividing elements often implicitly creates elements of the fraction
    field::

        sage: x = PolynomialRing(RationalField(), 'x').gen()
        sage: f = x/(x+1)
        sage: g = x**3/(x+1)
        sage: f/g
        1/x^2
        sage: g/f
        x^2

    The input must be an integral domain::

        sage: Frac(Integers(4))
        Traceback (most recent call last):
        ...
        TypeError: R must be an integral domain.
    """
    if not ring.is_Ring(R):
        raise TypeError("R must be a ring")
    if not R.is_integral_domain():
        raise TypeError("R must be an integral domain.")
    return R.fraction_field()


def is_FractionField(x):
    """
    Test whether or not ``x`` inherits from :class:`FractionField_generic`.

    EXAMPLES::

        sage: from sage.rings.fraction_field import is_FractionField
        sage: is_FractionField(Frac(ZZ['x']))
        True
        sage: is_FractionField(QQ)
        False
    """
    return isinstance(x, FractionField_generic)


class FractionField_generic(ring.Field):
    """
    The fraction field of an integral domain.
    """
    def __init__(self, R,
                 element_class=fraction_field_element.FractionFieldElement,
                 category=QuotientFields()):
        """
        Create the fraction field of the integral domain ``R``.

        INPUT:

        -  ``R`` -- an integral domain

        EXAMPLES::

            sage: Frac(QQ['x'])
            Fraction Field of Univariate Polynomial Ring in x over Rational Field
            sage: Frac(QQ['x,y']).variable_names()
            ('x', 'y')
            sage: category(Frac(QQ['x']))
            Category of quotient fields
        """
        self._R = R
        self._element_class = element_class
        Parent.__init__(self, base=R, names=R._names, category=category)

    def __reduce__(self):
        """
        For pickling.

        TESTS::

            sage: K = Frac(QQ['x'])
            sage: loads(dumps(K)) is K
            True
        """
        return FractionField, (self._R,)

    def _coerce_map_from_(self, S):
        """
        Return ``True`` if elements of ``S`` can be coerced into this
        fraction field.

        This fraction field has coercions from:

        - itself
        - any fraction field where the base ring coerces to the base
          ring of this fraction field
        - any ring that coerces to the base ring of this fraction field

        EXAMPLES::

            sage: F = QQ['x,y'].fraction_field()
            sage: F.has_coerce_map_from(F) # indirect doctest
            True

        ::

            sage: F.has_coerce_map_from(ZZ['x,y'].fraction_field())
            True

        ::

            sage: F.has_coerce_map_from(ZZ['x,y,z'].fraction_field())
            False

        ::

            sage: F.has_coerce_map_from(ZZ)
            True

        Test coercions::

            sage: F.coerce(1)
            1
            sage: F.coerce(int(1))
            1
            sage: F.coerce(1/2)
            1/2

        ::

            sage: K = ZZ['x,y'].fraction_field()
            sage: x,y = K.gens()
            sage: F.coerce(F.gen())
            x
            sage: F.coerce(x)
            x
            sage: F.coerce(x/y)
            x/y
            sage: L = ZZ['x'].fraction_field()
            sage: K.coerce(L.gen())
            x

        We demonstrate that :trac:`7958` is resolved in the case of
        number fields::

            sage: _.<x> = ZZ[]
            sage: K.<a> = NumberField(x^5-3*x^4+2424*x^3+2*x-232)
            sage: R = K.ring_of_integers()
            sage: S.<y> = R[]
            sage: F = FractionField(S)
            sage: F(1/a)
            (a^4 - 3*a^3 + 2424*a^2 + 2)/232

        Some corner cases have been known to fail in the past (:trac:`5917`)::

            sage: F1 = FractionField( QQ['a'] )
            sage: R12 = F1['x','y']
            sage: R12('a')
            a
            sage: F1(R12(F1('a')))
            a

            sage: F2 = FractionField( QQ['a','b'] )
            sage: R22 = F2['x','y']
            sage: R22('a')
            a
            sage: F2(R22(F2('a')))
            a

        Coercion from Laurent polynomials now works (:trac:`15345`)::

            sage: R = LaurentPolynomialRing(ZZ, 'x')
            sage: T = PolynomialRing(ZZ, 'x')
            sage: R.gen() + FractionField(T).gen()
            2*x
            sage: 1/(R.gen() + 1)
            1/(x + 1)

            sage: R = LaurentPolynomialRing(ZZ, 'x,y')
            sage: FF = FractionField(PolynomialRing(ZZ, 'x,y'))
            sage: prod(R.gens()) + prod(FF.gens())
            2*x*y
            sage: 1/(R.gen(0) + R.gen(1))
            1/(x + y)
        """
        from sage.rings.integer_ring import ZZ
        from sage.rings.rational_field import QQ
        from sage.rings.number_field.number_field_base import NumberField
        from sage.rings.polynomial.laurent_polynomial_ring import \
            LaurentPolynomialRing_generic

        if S is self._R:
            parent = self._R.Hom(self)
            return parent.__make_element_class__(FractionFieldEmbedding)(self._R, self, category=parent.homset_category())

        def wrapper(x):
            return self._element_class(self, x.numerator(), x.denominator())

        # The case ``S`` being `\QQ` requires special handling since `\QQ` is
        # not implemented as a ``FractionField_generic``.
        if S is QQ and self._R.has_coerce_map_from(ZZ):
            return CallableConvertMap(S, self, wrapper, parent_as_first_arg=False)

        # Number fields also need to be handled separately.
        if isinstance(S, NumberField):
            return CallableConvertMap(S, self,
                                      self._number_field_to_frac_of_ring_of_integers,
                                      parent_as_first_arg=False)

        # special treatment for LaurentPolynomialRings
        if isinstance(S, LaurentPolynomialRing_generic):
            def converter(x, y=None):
                if y is None:
                    return self._element_class(self, *x._fraction_pair())
                xnum, xden = x._fraction_pair()
                ynum, yden = y._fraction_pair()
                return self._element_class(self, xnum * yden, xden * ynum)
            return CallableConvertMap(S, self, converter, parent_as_first_arg=False)

        if (isinstance(S, FractionField_generic) and
                self._R.has_coerce_map_from(S.ring())):
            return CallableConvertMap(S, self, wrapper, parent_as_first_arg=False)

        if self._R.has_coerce_map_from(S):
            return CallableConvertMap(S, self, self._element_class,
                                      parent_as_first_arg=True)

        return None

    def _number_field_to_frac_of_ring_of_integers(self, x):
        r"""
        Return the number field element ``x`` as an element of ``self``,
        explicitly treating the numerator of ``x``  as an element of the ring
        of integers and the denominator as an integer.

        INPUT:

        -  ``x`` -- Number field element

        OUTPUT:

        -  Element of ``self``

        TESTS:

        We demonstrate that :trac:`7958` is resolved in the case of
        number fields::

            sage: _.<x> = ZZ[]
            sage: K.<a> = NumberField(x^5-3*x^4+2424*x^3+2*x-232)
            sage: R = K.ring_of_integers()
            sage: S.<y> = R[]
            sage: F = FractionField(S) # indirect doctest
            sage: F(1/a)
            (a^4 - 3*a^3 + 2424*a^2 + 2)/232
        """
        f = x.polynomial()   # Polynomial over QQ
        d = f.denominator()  # Integer
        return self._element_class(self, numerator=d * x, denominator=d)

    def is_field(self, proof=True):
        """
        Return ``True``, since the fraction field is a field.

        EXAMPLES::

            sage: Frac(ZZ).is_field()
            True
        """
        return True

    def is_finite(self):
        """
        Tells whether this fraction field is finite.

        .. NOTE::

            A fraction field is finite if and only if the associated
            integral domain is finite.

        EXAMPLES::

            sage: Frac(QQ['a','b','c']).is_finite()
            False

        """
        return self._R.is_finite()

    def base_ring(self):
        """
        Return the base ring of ``self``.

        This is the base ring of the ring
        which this fraction field is the fraction field of.

        EXAMPLES::

            sage: R = Frac(ZZ['t'])
            sage: R.base_ring()
            Integer Ring
        """
        return self._R.base_ring()

    def characteristic(self):
        """
        Return the characteristic of this fraction field.

        EXAMPLES::

            sage: R = Frac(ZZ['t'])
            sage: R.base_ring()
            Integer Ring
            sage: R = Frac(ZZ['t']); R.characteristic()
            0
            sage: R = Frac(GF(5)['w']); R.characteristic()
            5
        """
        return self._R.characteristic()

    def _repr_(self):
        """
        Return a string representation of ``self``.

        EXAMPLES::

            sage: Frac(ZZ['x']) # indirect doctest
            Fraction Field of Univariate Polynomial Ring in x over Integer Ring
        """
        return "Fraction Field of %s" % self._R

    def _latex_(self):
        """
        Return a latex representation of ``self``.

        EXAMPLES::

            sage: latex(Frac(GF(7)['x,y,z'])) # indirect doctest
            \mathrm{Frac}(\Bold{F}_{7}[x, y, z])
        """
        return "\\mathrm{Frac}(%s)" % latex.latex(self._R)

    def _magma_init_(self, magma):
        """
        Return a string representation of ``self`` in the given magma instance.

        EXAMPLES::

            sage: QQ['x'].fraction_field()._magma_init_(magma)            # optional - magma
            'SageCreateWithNames(FieldOfFractions(SageCreateWithNames(PolynomialRing(_sage_ref...),["x"])),["x"])'
            sage: GF(9,'a')['x,y,z'].fraction_field()._magma_init_(magma) # optional - magma
            'SageCreateWithNames(FieldOfFractions(SageCreateWithNames(PolynomialRing(_sage_ref...,3,"grevlex"),["x","y","z"])),["x","y","z"])'

        ``_magma_init_`` gets called implicitly below::

            sage: magma(QQ['x,y'].fraction_field())                  # optional - magma
            Multivariate rational function field of rank 2 over Rational Field
            Variables: x, y
            sage: magma(ZZ['x'].fraction_field())                    # optional - magma
            Univariate rational function field over Integer Ring
            Variables: x

        Verify that conversion is being properly cached::

            sage: k = Frac(QQ['x,z'])                                # optional - magma
            sage: magma(k) is magma(k)                               # optional - magma
            True
        """
        s = 'FieldOfFractions(%s)' % self.ring()._magma_init_(magma)
        return magma._with_names(s, self.variable_names())

    def ring(self):
        """
        Return the ring that this is the fraction field of.

        EXAMPLES::

            sage: R = Frac(QQ['x,y'])
            sage: R
            Fraction Field of Multivariate Polynomial Ring in x, y over Rational Field
            sage: R.ring()
            Multivariate Polynomial Ring in x, y over Rational Field
        """
        return self._R

    @cached_method
    def is_exact(self):
        """
        Return if ``self`` is exact which is if the underlying ring is exact.

        EXAMPLES::

            sage: Frac(ZZ['x']).is_exact()
            True
            sage: Frac(CDF['x']).is_exact()
            False
        """
        return self.ring().is_exact()

    def _element_constructor_(self, x, y=None, coerce=True):
        """
        Construct an element of this fraction field.

        EXAMPLES::

            sage: F = QQ['x,y'].fraction_field()
            sage: F._element_constructor_(1)
            1
            sage: F._element_constructor_(F.gen(0)/F.gen(1))
            x/y
            sage: F._element_constructor_('1 + x/y')
            (x + y)/y

        ::

            sage: K = ZZ['x,y'].fraction_field()
            sage: x,y = K.gens()

        ::

            sage: F._element_constructor_(x/y)
            x/y

        TESTS:

        The next example failed before :trac:`4376`::

            sage: K(pari((x + 1)/(x^2 + x + 1)))
            (x + 1)/(x^2 + x + 1)

        These examples failed before :trac:`11368`::

            sage: R.<x, y, z> = PolynomialRing(QQ)
            sage: S = R.fraction_field()
            sage: S(pari((x + y)/y))
            (x + y)/y

            sage: S(pari(x + y + 1/z))
            (x*z + y*z + 1)/z

        This example failed before :trac:`23664`::

            sage: P0.<x> = ZZ[]
            sage: P1.<y> = Frac(P0)[]
            sage: frac = (x/(x^2 + 1))*y + 1/(x^3 + 1)
            sage: Frac(ZZ['x,y'])(frac)
            (x^4*y + x^2 + x*y + 1)/(x^5 + x^3 + x^2 + 1)

        Test conversions where `y` is a string but `x` not::

            sage: K = ZZ['x,y'].fraction_field()
            sage: K._element_constructor_(2, 'x+y')
            2/(x + y)
            sage: K._element_constructor_(1, 'z')
            Traceback (most recent call last):
            ...
            TypeError: unable to evaluate 'z' in Fraction Field of Multivariate Polynomial Ring in x, y over Integer Ring

        Check that :trac:`17971` is fixed::

            sage: A.<a,c> = Frac(PolynomialRing(QQ,'a,c'))
            sage: B.<d,e> = PolynomialRing(A,'d,e')
            sage: R.<x> = PolynomialRing(B,'x')
            sage: (a*d*x^2+a+e+1).resultant(-4*c^2*x+1)
            a*d + 16*c^4*e + 16*a*c^4 + 16*c^4
        """
        if y is None:
            if isinstance(x, Element) and x.parent() is self:
                return x
            else:
                y = self.base_ring().one()

        try:
            return self._element_class(self, x, y, coerce=coerce)
        except (TypeError, ValueError):
            pass

        if isinstance(x, six.string_types):
            from sage.misc.sage_eval import sage_eval
            try:
                x = sage_eval(x, self.gens_dict_recursive())
            except NameError:
                raise TypeError("unable to evaluate {!r} in {}".format(x, self))
        if isinstance(y, six.string_types):
            from sage.misc.sage_eval import sage_eval
            try:
                y = sage_eval(y, self.gens_dict_recursive())
            except NameError:
                raise TypeError("unable to evaluate {!r} in {}".format(y, self))

        x = py_scalar_to_element(x)
        y = py_scalar_to_element(y)

        from sage.libs.pari.all import pari_gen
        if isinstance(x, pari_gen) and x.type() == 't_POL':
            # This recursive approach is needed because PARI
            # represents multivariate polynomials as iterated
            # univariate polynomials (see the above examples).
            # Below, v is the variable with highest priority,
            # and the x[i] are rational functions in the
            # remaining variables.
            v = self._element_class(self, x.variable(), 1)
            x = sum(self(x[i]) * v**i for i in range(x.poldegree() + 1))

        while True:
            x0, y0 = x, y
            try:
                x = x0.numerator()*y0.denominator()
                y = y0.numerator()*x0.denominator()
            except AttributeError:
                raise TypeError("cannot convert {!r}/{!r} to an element of {}",
                                x0, y0, self)
            try:
                return self._element_class(self, x, y, coerce=coerce)
            except TypeError:
                if not x != x0:
                    raise

    def construction(self):
        """
        EXAMPLES::

            sage: Frac(ZZ['x']).construction()
            (FractionField, Univariate Polynomial Ring in x over Integer Ring)
            sage: K = Frac(GF(3)['t'])
            sage: f, R = K.construction()
            sage: f(R)
            Fraction Field of Univariate Polynomial Ring in t over Finite Field of size 3
            sage: f(R) == K
            True
        """
        from sage.categories.pushout import FractionField
        return FractionField(), self.ring()

    def __eq__(self, other):
        """
        Check whether ``self`` is equal to ``other``.

        EXAMPLES::

            sage: Frac(ZZ['x']) == Frac(ZZ['x'])
            True
            sage: Frac(ZZ['x']) == Frac(QQ['x'])
            False
            sage: Frac(ZZ['x']) == Frac(ZZ['y'])
            False
            sage: Frac(ZZ['x']) == QQ['x']
            False
        """
        if not isinstance(other, FractionField_generic):
            return False
        return self._R == other._R

    def __ne__(self, other):
        """
        Check whether ``self`` is not equal to ``other``.

        EXAMPLES::

            sage: Frac(ZZ['x']) != Frac(ZZ['x'])
            False
            sage: Frac(ZZ['x']) != Frac(QQ['x'])
            True
            sage: Frac(ZZ['x']) != Frac(ZZ['y'])
            True
            sage: Frac(ZZ['x']) != QQ['x']
            True
        """
        return not (self == other)

    def ngens(self):
        """
        This is the same as for the parent object.

        EXAMPLES::

            sage: R = Frac(PolynomialRing(QQ,'z',10)); R
            Fraction Field of Multivariate Polynomial Ring in z0, z1, z2, z3, z4, z5, z6, z7, z8, z9 over Rational Field
            sage: R.ngens()
            10
        """
        return self._R.ngens()

    def gen(self, i=0):
        """
        Return the ``i``-th generator of ``self``.

        EXAMPLES::

            sage: R = Frac(PolynomialRing(QQ,'z',10)); R
            Fraction Field of Multivariate Polynomial Ring in z0, z1, z2, z3, z4, z5, z6, z7, z8, z9 over Rational Field
            sage: R.0
            z0
            sage: R.gen(3)
            z3
            sage: R.3
            z3
        """
        x = self._R.gen(i)
        one = self._R.one()
        r = self._element_class(self, x, one, coerce=False, reduce=False)
        return r

    def _is_valid_homomorphism_(self, codomain, im_gens):
        """
        Check if the homomorphism defined by sending generators of this
        fraction field to ``im_gens`` in ``codomain`` is valid.

        EXAMPLES::

            sage: F = QQ['x,y'].fraction_field()
            sage: x,y = F.gens()
            sage: F._is_valid_homomorphism_(F, [y,x])
            True
            sage: R = ZZ['x']; x = R.gen()
            sage: F._is_valid_homomorphism_(R, [x, x])
            False

        TESTS::

            sage: F._is_valid_homomorphism_(ZZ, [])
            False

        Test homomorphisms::

            sage: phi = F.hom([2*y, x])
            sage: phi(x+y)
            x + 2*y
            sage: phi(x/y)
            2*y/x
        """
        if len(im_gens) != self.ngens():
            return False
        # it is enough to check if elements of the base ring coerce to
        # the codomain
        if codomain.has_coerce_map_from(self.base_ring()):
            return True
        return False

    def random_element(self, *args, **kwds):
        """
        Return a random element in this fraction field.

        The arguments are passed to the random generator of the underlying ring.

        EXAMPLES::

            sage: F = ZZ['x'].fraction_field()
            sage: F.random_element()  # random
            (2*x - 8)/(-x^2 + x)

        ::

            sage: f = F.random_element(degree=5)
            sage: f.numerator().degree()
            5
            sage: f.denominator().degree()
            5
        """
        return self._element_class(self, self._R.random_element(*args, **kwds),
                                   self._R._random_nonzero_element(*args, **kwds),
                                   coerce=False, reduce=True)


    def some_elements(self):
        r"""
        Return some elements in this field.

        EXAMPLES::

            sage: R.<x> = QQ[]
            sage: R.fraction_field().some_elements()
            [0,
             1,
             x,
             2*x,
             x/(x^2 + 2*x + 1),
             1/x^2,
             ...
             (2*x^2 + 2)/(x^2 + 2*x + 1),
             (2*x^2 + 2)/x^3,
             (2*x^2 + 2)/(x^2 - 1),
             2]

        """
        ret = [self.zero(), self.one()]
        for a in self._R.some_elements():
            for b in self._R.some_elements():
                if a != b and self(a) and self(b):
                    ret.append(self(a)/self(b))
        return ret


class FractionField_1poly_field(FractionField_generic):
    """
    The fraction field of a univariate polynomial ring over a field.

    Many of the functions here are included for coherence with number fields.
    """
    def __init__(self, R,
                 element_class=fraction_field_element.FractionFieldElement_1poly_field):
        """
        Just change the default for ``element_class``.

        EXAMPLES::

            sage: R.<t> = QQ[]; K = R.fraction_field()
            sage: K._element_class
            <class 'sage.rings.fraction_field_element.FractionFieldElement_1poly_field'>
        """
        FractionField_generic.__init__(self, R, element_class)

    def ring_of_integers(self):
        """
        Return the ring of integers in this fraction field.

        EXAMPLES::

            sage: K = FractionField(GF(5)['t'])
            sage: K.ring_of_integers()
            Univariate Polynomial Ring in t over Finite Field of size 5
        """
        return self._R

    def maximal_order(self):
        """
        Return the maximal order in this fraction field.

        EXAMPLES::

            sage: K = FractionField(GF(5)['t'])
            sage: K.maximal_order()
            Univariate Polynomial Ring in t over Finite Field of size 5
        """
        return self._R

    def class_number(self):
        """
        Here for compatibility with number fields and function fields.

        EXAMPLES::

            sage: R.<t> = GF(5)[]; K = R.fraction_field()
            sage: K.class_number()
            1
        """
        return 1

    def _factor_univariate_polynomial(self, f):
        r"""
        Return the factorization of ``f`` over this field.

        EXAMPLES::

            sage: k.<a> = GF(9)
            sage: K = k['t'].fraction_field()
            sage: R.<x> = K[]
            sage: f = x^3 + a
            sage: f.factor()
            (x + 2*a + 1)^3

        """
        # The default implementation would try to convert this element to singular and factor there.
        # This fails silently over some base fields, see #23642, so we convert
        # to the function field and factor there.
        return f.change_ring(self.function_field()).factor().base_change(f.parent())

    def function_field(self):
        r"""
        Return the isomorphic function field.

        EXAMPLES::

            sage: R.<t> = GF(5)[]
            sage: K = R.fraction_field()
            sage: K.function_field()
            Rational function field in t over Finite Field of size 5

        .. SEEALSO::

            :meth:`sage.rings.function_field.RationalFunctionField.field`

        """
        from sage.rings.all import FunctionField
        return FunctionField(self.base_ring(), names=self.variable_name())

    def _coerce_map_from_(self, R):
        r"""
        Return a coerce map from ``R`` to this field.

        EXAMPLES::

            sage: R.<t> = GF(5)[]
            sage: K = R.fraction_field()
            sage: L = K.function_field()
            sage: f = K.coerce_map_from(L); f # indirect doctest
            Isomorphism morphism:
              From: Rational function field in t over Finite Field of size 5
              To:   Fraction Field of Univariate Polynomial Ring in t over Finite Field of size 5
            sage: f(~L.gen())
            1/t

        """
        from sage.rings.function_field.function_field import is_RationalFunctionField
        if is_RationalFunctionField(R) and self.variable_name() == R.variable_name() and self.base_ring() is R.constant_base_field():
            from sage.categories.all import Hom
            parent = Hom(R, self)
            from sage.rings.function_field.maps import FunctionFieldToFractionField
            return parent.__make_element_class__(FunctionFieldToFractionField)(parent)

        return super(FractionField_1poly_field, self)._coerce_map_from_(R)


class FractionFieldEmbedding(DefaultConvertMap_unique):
    r"""
    The embedding of an integral domain into its field of fractions.

    EXAMPLES::

        sage: R.<x> = QQ[]
        sage: f = R.fraction_field().coerce_map_from(R); f
        Coercion map:
          From: Univariate Polynomial Ring in x over Rational Field
          To:   Fraction Field of Univariate Polynomial Ring in x over Rational Field
 
    TESTS::

        sage: from sage.rings.fraction_field import FractionFieldEmbedding
        sage: isinstance(f, FractionFieldEmbedding)
        True
        sage: TestSuite(f).run()

    Check that :trac:`23185` has been resolved::

        sage: R.<x> = QQ[]
        sage: K.<x> = FunctionField(QQ)
        sage: R.is_subring(K)
        True
        sage: R.is_subring(R.fraction_field())
        True

    """
    def is_surjective(self):
        r"""
        Return whether this map is surjective.

        EXAMPLES::

            sage: R.<x> = QQ[]
            sage: R.fraction_field().coerce_map_from(R).is_surjective()
            False

        """
        return self.domain().is_field()

    def is_injective(self):
        r"""
        Return whether this map is injective.

        EXAMPLES:

        The map from an integral domain to its fraction field is always
        injective:

            sage: R.<x> = QQ[]
            sage: R.fraction_field().coerce_map_from(R).is_injective()
            True

        """
        return True

    def section(self):
        r"""
        Return a section of this map.

        EXAMPLES::

            sage: R.<x> = QQ[]
            sage: R.fraction_field().coerce_map_from(R).section()
            Section map:
              From: Fraction Field of Univariate Polynomial Ring in x over Rational Field
              To:   Univariate Polynomial Ring in x over Rational Field

        """
        from sage.categories.sets_with_partial_maps import SetsWithPartialMaps
        from sage.all import Hom
        parent = Hom(self.codomain(), self.domain(), SetsWithPartialMaps())
        return parent.__make_element_class__(FractionFieldEmbeddingSection)(self)

    def _richcmp_(self, other, op):
        r"""
        Compare this element to ``other`` with respect to ``op``.

        EXAMPLES::

            sage: R.<x> = QQ[]
            sage: f = R.fraction_field().coerce_map_from(R)
            sage: S.<y> = GF(2)[]
            sage: g = S.fraction_field().coerce_map_from(S)
 
            sage: f == g # indirect doctest
            False
            sage: f == f
            True

        """
        if type(self) != type(other):
            return NotImplemented
        return richcmp((self.domain(), self.codomain()), (other.domain(), other.codomain()), op)

    def __hash__(self):
        r"""
        Return a hash value for this embedding.

        EXAMPLES::

            sage: R.<x> = QQ[]
            sage: hash(R.fraction_field().coerce_map_from(R)) == hash(R.fraction_field().coerce_map_from(R))
            True

        """
        return hash((type(self), self.domain()))


class FractionFieldEmbeddingSection(Section):
    r"""
    The section of the embedding of an integral domain into its field of
    fractions.

    EXAMPLES::

        sage: R.<x> = QQ[]
        sage: f = R.fraction_field().coerce_map_from(R).section(); f
        Section map:
          From: Fraction Field of Univariate Polynomial Ring in x over Rational Field
          To:   Univariate Polynomial Ring in x over Rational Field
 
    TESTS::

        sage: from sage.rings.fraction_field import FractionFieldEmbeddingSection
        sage: isinstance(f, FractionFieldEmbeddingSection)
        True
        sage: TestSuite(f).run()

    """
    def _call_(self, x, check=True):
        r"""
        Evaluate this map at ``x``.

        EXAMPLES::

            sage: R.<x> = QQ[]
            sage: K = R.fraction_field()
            sage: x = K.gen()
            sage: f = K.coerce_map_from(R).section()
            sage: f(x)
            x
            sage: f(1/x)
            Traceback (most recent call last):
            ...
            TypeError: fraction must have unit denominator

        TESTS:

        Over inexact rings, we have to take the precision of the denominators
        into account::

            sage: R=ZpCR(2)
            sage: S.<x> = R[]
            sage: f = x/S(R(3,absprec=2))
            sage: S(f)
            (1 + 2 + O(2^2))*x

        """
        if self.codomain().is_exact() and x.denominator().is_one():
           return x.numerator()
        if check and not x.denominator().is_unit():
            # This should probably be a ValueError.
            # However, too much existing code is expecting this to throw a
            # TypeError, so we decided to keep it for the time being.
            raise TypeError("fraction must have unit denominator")
        return x.numerator() * x.denominator().inverse_of_unit()

    def _call_with_args(self, x, args=(), kwds={}):
        r"""
        Evaluation this map at ``x``.

        INPUT:

        - ``check`` -- whether or not to check

        EXAMPLES::

            sage: R.<x> = QQ[]
            sage: K = R.fraction_field()
            sage: R(K.gen(), check=True)
            x

        """
        check = kwds.pop('check', True)
        if args or kwds:
            raise NotImplementedError("__call__ can not be called with additional arguments other than check=True/False")
        return self._call_(x, check=check)

    def _richcmp_(self, other, op):
        r"""
        Compare this element to ``other`` with respect to ``op``.

        EXAMPLES::

            sage: R.<x> = QQ[]
            sage: f = R.fraction_field().coerce_map_from(R).section()
            sage: S.<y> = GF(2)[]
            sage: g = S.fraction_field().coerce_map_from(S).section()
 
            sage: f == g # indirect doctest
            False
            sage: f == f
            True

        """
        if type(self) != type(other):
            return NotImplemented
        return richcmp((self.domain(), self.codomain()), (other.domain(), other.codomain()), op)

    def __hash__(self):
        r"""
        Return a hash value for this section.

        EXAMPLES::

            sage: R.<x> = QQ[]
            sage: hash(R.fraction_field().coerce_map_from(R).section()) == hash(R.fraction_field().coerce_map_from(R).section())
            True

        """
        return hash((type(self), self.codomain()))
