Type Checking

Type-checking Operators allow a value's type to be checked. As well as actual types, there are also "pseudo-types" that can be checked for.

Symbol	Type	Name	Description	Example
typeof	Prefix	TypeOf	Returns the actual type of the term's value	typeof a
is	Infix	IsType	Checks if the value of the left term is of the actual type or pseudo-type specified by the type descriptor in the right term	a is int
is not	Infix	IsNotType	Checks if the value of the left term is not of the actual type or pseudo-type specified by the type descriptor in the right term	a is not int
is a	Infix	IsType	Optional form of is	a is a string
is not a	Infix	IsNotType	Optional form of is not	a is not a string
is an	Infix	IsType	Optional form of is	a is an integer
is not an	Infix	IsNotType	Optional form of is not	a is not an integer

is not is a special form of is that allows the inverse to be checked in a convenient manner without using parentheses.

For example:

{ a is not string }
{ !(a is string) }

Both expressions in the example above are equivalent.

Optionally, the words a and an can be used with is and is not, purely to make the expression more readable.

For example:

{ a is not string }
{ a is not a string }

Both expressions in the example above are equivalent.

There are two kinds of type descriptor - those that describe actual types, which are real data-types; and those that describe "pseudo-types", which are not data types at all, but additional types that are useful to use with the is Operator.

Actual types
array	Comparison is true if the value is an Array
boolean or bool	Comparison is true if the value is a Boolean
float	Comparison is true if the value is a floating-point Number
integer or int	Comparison is true if the value is an integer Number
number	Comparison is true if the value is a Number, whether integer or floating-point
numeric	Comparison is true if the value is a Number, or a String containing a number
string	Comparison is true if the value is a String
Pseudo-types
function	Comparison is true if the value is the name of a function that exists
macro	Comparison is true if the value is the name of a macro that exists
section	Comparison is true if the value is the name of a section that exists
validfile or validinclude or readablefile or readableinclude	Comparison is true if the value is the name of a file that is allowed to be included by the Include command, exists, and is readable
existentfile or existentinclude	Comparison is true if the value is the name of a file that is allowed to be included by the Include command, and exists, without checking readability
allowedfile or allowedinclude	Comparison is true if the value is the name of a file that is allowed to be included by the Include command, without checking existence or readability