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_E_x_p_r_e_s_s_i_o_n_s
In Miranda an expression denotes a value. Expressions occur in two
contexts, but have the same(*) syntax in both situations. First, the
basic mode of operation of the Miranda command interpreter is that it
evaluates expressions typed at the terminal (these are called
`command-level expressions'). Second, expressions are an important
syntactic component of _s_c_r_i_p_t_s (because scripts are collections of
definitions, and the right hand sides of definitions are composed of
expressions).
Expressions typed at the terminal are volatile - only by being made part
of a script can an expression be saved for future use.
An expression is either simple, or a function application, or an
operator expression, or an operator.
_A_ _s_i_m_p_l_e_ _e_x_p_r_e_s_s_i_o_n_ _i_s_ _o_n_e_ _o_f_ _t_h_e_ _f_o_l_l_o_w_i_n_g:
Identifier: (see separate manual entry) these are of two kinds based on
the case of the initial letter
- variable e.g. `x' or `map'
- constructor e.g. `X' or `True'
Literal, e.g. 136 or "fruit" (see separate manual entry)
An operator section (see separate manual entry)
The keyword `_s_h_o_w' or `_r_e_a_d_v_a_l_s' (see separate manual entries)
A list, such as `[1,5,7,9]' or a `dotdot' list or a list comprehension
(see manual entry on iterative expressions).
A tuple, such as `(True,"green",37)'. Tuples differ from lists in that
they can have components of mixed type. They are always enclosed in
round parentheses. The empty tuple, which has no components, is written
`()'. Otherwise, a tuple must have at least two components - there is
no concept of a one-tuple. Tuples cannot be subscripted, but their
components can be extracted by pattern matching. Since there is no
concept of a one-tuple, the use of parentheses for grouping does not
conflict with their use for tuple formation.
Any expression enclosed in parentheses is a simple expression.
_F_u_n_c_t_i_o_n_ _a_p_p_l_i_c_a_t_i_o_n
is denoted by juxtaposition, and is left associative, so e.g.
f a b
denotes a curried function application of two arguments. (So f is
really a function of one argument whose result is another function -
thus `f a b' is actually parsed as `(f a) b' - the advantage of this
arrangement is that `f a' has a meaning in its own right, it is a
partially applied version of f.)
_O_p_e_r_a_t_o_r_ _e_x_p_r_e_s_s_i_o_n_s
e.g. `5*x-17'
There are a variety of infix and prefix operators, of various binding
powers (see manual entry on operators). Note that function application
is more binding than any operator.
An operator on its own can be used as the name of the corresponding
function, as shown in the following examples
arith2_ops = [+,-,*,/,div,mod,^]
sum = foldr (+) 0
both of which are legal definitions. Note that when an operator is
passed as a parameter it needs to be parenthesised, to force the correct
parse. An ambiguity arises in the case of `-' which has two meanings as
an operator (infix and prefix) - the convention is that `-' occurring
alone always refers to the infix (i.e. dyadic) case. The name `neg' is
provided for the unary minus function, as part of the standard
environment.
A formal syntax for expressions can be found in the manual section
called `Formal syntax of Miranda scripts'.
(*) _N_o_t_e There is one exception to the rule that command level
expressions have the same syntax as expressions inside scripts - the
notation `$$', meaning the last expression evaluated, is allowed only in
command level expressions.
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