Editing BLISS (section)

== Language description ==
{{long quote|date=June 2020}}
{{quote|BLISS has many of the features of other modern high-level languages. It has block structure, an automatic stack, and mechanisms for defining and calling recursive routines&nbsp;... provides a variety of predefined data structures and&nbsp;... facilities for testing and iteration&nbsp;...

On the other hand, BLISS omits certain features of other high-level languages. It does not have built-in facilities for input/output, because a system-software project usually develops its own input/output or builds on basic monitor I/O or screen management services&nbsp;... it permits access to machine-specific features, because system software often requires this. BLISS has characteristics that are unusual among high-level languages. A name&nbsp;... is uniformly interpreted as the address of that segment rather than the value of the segment&nbsp;... Also, BLISS is an "expression language" rather than a "statement language".
This means that every construct of the language that is not a declaration is an expression. Expressions produce a value as well as possibly causing an action such as modification of storage, transfer of control, or execution of a program loop. For example, the counterpart of an assignment "statement" in BLISS is, strictly speaking, an expression that itself has a value. The value of an expression can be either used or discarded in BLISS&nbsp;... Finally, BLISS includes a macro facility that provides a level of capability usually found only in macro-assemblers.|''Bliss Language Manual'', [[Digital Equipment Corporation]] (1987)<ref name=Manual>[http://www.digiater.nl/openvms/freeware/v80/bliss/documentation/blslref.pdf Bliss Language Manual], [[Digital Equipment Corporation]] (1987)</ref>}}

The BLISS language has the following characteristics:

* All ''constants'' are full word for the machine being used, e.g. on a 16-bit machine such as the [[PDP-11]], a constant is 16 bits; on a [[VAX]] computer, constants are 32 bits, and on a [[PDP-10]], a constant is 36 bits.
* A ''reference to a variable'' is always to the address of that variable. For example, the instruction <code>Z+8</code> refers to adding 8 to the ''address'' of Z, not to its ''value''. If one needs to add 8 to the ''value'' of Z, one must prefix the variable with a period; so one would type <code>.Z+8</code> to perform this function, which adds 8 to the ''contents'' of Z.
* ''Assignment'' is done with the standard <code>=</code> symbol, e.g. <code>Z=8</code> &ndash; which says to create a full-word constant containing 8, and store it in the location whose address corresponds to that of Z. So <code>Z+12=14</code> (or, alternatively <code>12+Z=14</code> though this form is considered bad practice per the BLISS documentation in Digital) places the constant 14 into the location which is 12 words after the address of Z.
* ''Block statements'' are similar to those of [[ALGOL]]: a block is started with a <code>BEGIN</code> statement and terminated with <code>END</code>. As with ALGOL, statements are separated with the semicolon (";"). When a value is computed, it is saved until the next statement terminator &ndash; which means that a value can be computed, assigned to a variable, and carried forward to the next statement, if desired. Alternatively, an open parenthesis may be used to begin a block, with the close parenthesis used to close the block. When parentheses are included in an expression, the standard [[Order of operations|precedence]] rules are used, in which parenthesized expressions are computed first,
* ''Conditional execution'' uses the <code>IF</code> expression, which tests a true-false condition, performs alternative actions, and returns a result.
* ''Comparison'' uses keywords such as <code>EQL</code> for equality (as opposed to [[operator overloading|overloading]] the = symbol for the same purpose), <code>GTR</code> for Greater Than, and <code>NEQ</code> for not equal. For example, the following code will assign the absolute value of Z to the address indicated by Q:

::<code>Q = (IF .Z GTR 0 THEN .Z ELSE -.Z);</code>

* ''Identifiers'' (variables and constants) must be declared before use, typically using the <code>OWN</code> keyword. Declaring a variable normally causes the compiler to allocate space for it; when necessary, a variable may be assigned a fixed machine address via the <code>BIND</code> declaration. This feature is primarily used for accessing either machine registers or certain special addresses.
* ''[[Subroutine]]s'' in the language are called ''routines'', and are declared with the keyword <code>ROUTINE</code>.
* ''Macros'', which allow for text substitution, are declared with the keyword <code>MACRO</code>.
* The language supports ''[[Array data structure|arrays]]'', which are referred to as ''structures'', and declared with the keyword <code>VECTOR</code>.
* The language supports some ''[[high-level programming language]] constructs'' such as:
**''Alternative execution paths'' via the <code>CASE</code> expression
**''Looping'' through use of the <code>INCR</code> expression, which is similar to ALGOL's FOR statement
**Built-in ''[[String (computer science)|string]] functions''
**Certain automatic ''data conversions'' (number to string, etc.)