diff options
Diffstat (limited to 'miralib/manual/7')
| -rw-r--r-- | miralib/manual/7 | 81 |
1 files changed, 81 insertions, 0 deletions
diff --git a/miralib/manual/7 b/miralib/manual/7 new file mode 100644 index 0000000..bd9e102 --- /dev/null +++ b/miralib/manual/7 @@ -0,0 +1,81 @@ +_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. + |