| Data Type | Description | Sample Values |
| atom | "Abstract Points" with no identifying properties except their existence. The keyword newat has as its value an atom never before seen in this session of ISETL. | |
| boolean | true or false | true false |
| file | An external file created by applying one of the pre-defined functions openr, opena, openw | |
| floating point | A floating point constant is a sequence of zero or more digits, followed by a decimal point, followed by zero or more digits. A floating point constant may be followed by an exponent. An exponent consists of one of the characters "e" or "E" followed by a signed or unsigned integer. The value of a floating point constant is determined as in scientific notation. The possible range of floating point numbers is machine dependent, but at a minimum should have 5 place accuracy. | 5.7 4.37e-4 .3 |
| function | A Subroutine that returns a value. | >f := func(x); >>return x*x; >end; |
| integer | An integer constant is a sequence of one or more digits. It represents an unsigned integer. There is no limit to the size of integers. | 37 12345678901234567890 |
| map | A map is a set that is either empty or whose elements are all ordered pairs. An ordered pair is a tuple whose first two components and no others are defined. A map in which no value appears more than once as the first component of an ordered pair is called a single-valued map or smap; otherwise, the map is called a multi-valued map or mmap. | {[1,1], [2,4], [3,9]} |
| mmap | See map. | |
| om (OM) | undefined | om OM |
| procedure | A Subroutine that doesn't return a value. | >p := proc(); >>writeln "Hi"; >end; |
| rational | A fraction | >3/4; 0.750; >!rational on >3/4; 3/4; > |
| real | See floating point | |
| set | An unordered list of elements enclosed in braces, { and }, and separated by commas. The elements may be of any type, mixed heterogeneously, even another set. Elements occur at most once per set. OM may not be an element of a set. The order of elements is not significant in a set and printing (displaying) the value of a set twice in succession could display the elements in different orders. As a special case the empty set is denoted by { }. | {1,3,42,9} {15.2,"Hi",{5,9},false,[3,7]} |
| smap | See map. | |
| string | A string constant is any sequence of characters surrounded by double quotes or single quotes. | "This is a string." |
| tuple | An ordered list of components enclosed in square brackets, [ and ], and separated by commas. | [5,9,false,13] |