┌────────────────────────────────────┐
                    │      "Declarative languages,       │
                    │ have no notion of execution state, │
                    │   nor of the passing of time."     │
                    └────────────────────────────────────┘

Functional  programming  of  which  (nicely written) D-functions are an example,
derives  from  the  slightly  more  abstract  notion of declarative programming.
http://www.foldoc.org gives the following definition for "Declarative Language":

"A  general  term for a relational language or a functional language, as opposed
 to an imperative language. Imperative (or procedural) languages specify explic-
 it sequences of steps to follow to produce a result, while declarative languag-
 es  describe relationships between variables in terms of functions or inference
 rules  and  the  language executor (interpreter or compiler) applies some fixed
 algorithm  to  these relations to produce a result. The most common examples of
 declarative languages are logic programming languages such as Prolog and funct-
 ional languages like Haskell."

The scripts in this workspace can be seen as programs in such a language. From a
background  in  procedural languages such as C or traditional APL, it might seem
surprising that for example, an algorithm for long division could be written us-
ing only equations. The algorithm needs no variables (in the procedural sense of
the  word, as nothing  varies) and has no procedural control structures, such as
loops or conditional execution (see: Long_division).

In  this  context,the term "pattern variable" used in these scripts to declare a
set  of  pattern-matching  values, is a misnomer. Perhaps "pattern set" would be
more appropriate.

Here is (one view of) a partial taxonomy of computer languages:

                                   Language
                    ┌─────────────────┴───────────────────────┐
                Declarative                               Procedural
           ┌────────┴─────────────────────┬───────┬─···  ┌────┴────┬────···
       Functional                       Logic     ?      C     Trad_APL
       ┌───┴───┬────┬────┬─···       ┌────┴─···   │
    Haskell    D   min  max        Prolog     eval_script

See also: dfns.dws/notes.pearly

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