For classes and methods with many parameters it may sometimes be difficult to remember which parameter is which in the list.
To improve readability it is possible to associate a name with a parameter – such a name is called a fat comma since it is used instead of a comma.
We have seen examples of using fat comma for method invocations and one such example is the put method of the Array class introduced in section .
a: obj Array(100,integer)
a.put(7):at[3]The statement a.put(7):at[3] describes the invocation of a method put:at with two arguments 7 and 3. This method assigns the value 7 at index 3 in the array a.
If we instead just use traditional comma-based syntax, this statement would have to written like:
a.put(7,3) With this syntax it may not be clear whether the index is the first or second parameter. Using the fat comma syntax, this is not a problem.
The overall structure of the Array class is as follows:
class Array(range: var integer, element:< Object):
get[inx: var integer]:
:::
put(e: var element):at[inx: var integer]:
:::Class Array has two parameters range, the number of elements of the Array and element, the type of the elements in the Array. It has two attributes, the method get and the method put:at, which are described in section
We have seen other examples of using fat comma such as the control method if:then, if:then:else, and for:to:repeat. We show the overall structure of if:then:else below in this section and a more detailed description is given in section .
The general form of declaring a method using fat comma is:
Id1(parameter1):id2(paramater2):id3(parameter3):
...This method has the name Id1:id2:id3 and has three parameters specified by parameter1, parameter2, and parameter3.
An invocation has the form:
Id1(exp1):id2(exp2):id3(exp3)where exp1, exp2, and exp3 are the arguments being supplied.
There may be an arbitrary number of parameters – for brevity, we have just shown 3 parameters.
It is possible to use different parentheses like (, ), [, ], {, and } as in:
put(e):at[inx]:do{S}By convention, (, and ) are used for a parameter representing a datum in general, [, and ] are used for a parameter representing an index, and {, and } are used for a parameter being a virtual method or class. However, these are not rules enforces by the compiler and the programmer may use whatever parameters he/she prefers, but we recommend following the conventions.
For a parameter defined using curly brackets {, and }, these may in an invocation be replaced by indentation of the actual argument.
Consider the if:then:else, which has the following overall structure:
if (cond):then{thenPart:< object}else{ elsePart:< Object}:
::: An if:then:else may be invoked as follows using curly brackets:
if (a < b) :then { a := a - b }:else { b := b - a }If indentation is used instead of the curly brackets, this if:then:else may be written as follows:
if (a < b) :then
a := a - b
:else
b := b - aFat comma may be used to specify the parameters of classes as well as methods.
