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_E_m_p_t_y_ _t_y_p_e_s (also called _P_l_a_c_e_h_o_l_d_e_r_ _t_y_p_e_s)
An empty type has no values belonging to it (apart from the undefined
value, undef, which is a member of every type). Empty types are
declared as follows:
widget :: _t_y_p_e
this declares `widget' to be a type but gives it no values.
Empty types can be used during program development as placeholders for
types whose representation is not yet decided. For example given the
above declaration we can give type specifications involving widget, e.g.
foo :: num->widget->widget
gronk :: [widget]->widget
and write code using `foo' and `gronk' which can be checked for type
correctness. At a later stage the specification of widget as an empty
type can be replaced by a non-empty type definition, using ==, ::=, or
_a_b_s_t_y_p_e, allowing foo, gronk, to be defined.
Typenames declared as empty can have any arity, eg
table * ** :: _t_y_p_e
This creates a family of empty types, such as `table num bool' and so
on. They are all devoid of values (apart from undef). The general form
of this kind of specification is
tform-list :: _t_y_p_e
where `tform' consists of a typename followed by zero or more generic
type variables (and it is permitted to declare several such types
simultaneously, separated by commas, whence `tform-list').
An empty type may be considered equivalent to an algebraic type with no
constructors.
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