Expressions

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 scripts (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 simple expression is one of the following:

    Identifier, e.g. `x` (see separate manual entry)

    Literal, e.g. `136` (see separate manual entry)

    An operator section (see separate manual entry)

    The keyword `show' or `readvals` (see separate manual entries)

    A list, such as `[1,5,7,9]' or a `dotdot` list or a  list  comprehension
    (see separate 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.

Function application 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.)

Operator expressions 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.

(*) Note 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.