Sections

An infix operator can be partially applied, by supplying  it  with  only
one  of its operands, the result being a function of one argument.  Such
expressions are always parenthesised, to avoid ambiguity, and are called
`sections'.   Two  different  kinds  of sections (called presections and
postsections) are possible since either the first or the second  operand
can be supplied.

An example of a presection is
	(1/)
which denotes the reciprocal function.  An example of a postsection is
	(/3)
which gives a concise notation for the `divide by three' function.  Note
that  both of these examples are functions of type (num->num).  With one
exception (see below) sections can be formed using any  infix  operator.
Further  examples  are  (0:)  which is a function for prefixing lists of
numbers with zero, and (^2) which is the square function.

Sections  may  be  regarded  as  the  analogue  of  currying  for  infix
operators.  They are a minor syntactic convenience, and  do  not  really
add  any  power  to the language, since any function denoted in this way
could have been introduced by explicit definition.  For  the  first  two
examples given above we could have written, say
	reciprocal y = 1/y
	divide_by_three x = x/3
and then used the function names, although this would have been somewhat
more verbose.

To summarise the conventions for infixes, taking infix  division  as  an
example, note that the following expressions are all equivalent.
	a / b
	(/) a b
	(a /) b
	(/ b) a

The usual  rules  about  operator  precedence  (see  manual  section  on
operators)  apply  to sections.  For example we can write (a*b+) but not
(a+b*), because `*' is more binding than `+'.  The latter example should
have   been  written  ((a+b)*).   As  always  when  writing  complicated
expressions, if there is any possibility of ambiguity it  is  better  to
put in extra parentheses.

Special case
 It is not possible to form a postsection in infix minus, because  of  a
conflict of meaning with unary minus.  For example:
	(-b)
is  a  parenthesised  occurrence of negative b, not a section.  As a way
round this there is a function `subtract' in  the  standard  environment
with  the  definition:-  subtract x y = y - x.  This is a normal curried
function, so we  can  write  (subtract  b)  to  get  the  function  that
subtracts b from things.

Presections in infix minus, such as (a-), cause no ambiguity.  There are
no  problems  with infix plus because Miranda does not have a unary plus
operator.

Acknowledgement:
 The idea of sections is due  to  Richard  Bird  (of  Oxford  University
Programming  Research Group) and David Wile (of USC Information Sciences
Institute).

