Hi Folks,
Below is a summary of our discussions. Please let me
know where I err.
Declarative programming is a style of programming.
You can write declarative programs in any programming language. However, some
programming languages are designed to nudge you into the declarative mindset;
programming languages that don’t have assignment statements fall into
this category, e.g., XSLT.
To illustrate the declarative mindset, consider
this:
Area = Length *
Breadth (1)
It is describing a relationship between Area,
Length, and Breadth. It is not an assignment statement or instruction. It is
declaratively stating, “Here, this is a relationship that will be used in
the program.”
Consider this:
Carpet_Cost = Area *
Price_Per_Unit (2)
It is also a description of a relationship. It
builds on top of the previous relationship. Thus, a larger concept
(Carpet_Cost) is described using a smaller concept (Area). This is sometimes
known as “functional composition.”
It is important to emphasize that (1) and (2) are
descriptions; they are not assignment statements/instructions.
Area, Length, Breadth, Carpet_Cost, Price_Per_Unit
are called variables. The term “variable” is used in the sense,
“This is a symbol used to represent any (numeric) value.”
Interestingly, this is also how mathematics uses the term.
Definite values may be given to variables; thus, if
Length is given the value 6, Breadth is given 5, and Price_Per_Unit is given 2
then execution of this declarative program:
Area = Length * Breadth
Carpet_Cost = Area *
Price_Per_Unit
Yields these results:
- Area is 30
- Carpet_Cost is 60
Relationships may apply to multiple things, e.g.,
For each House_Room:
Area = Length *
Breadth
Carpet_Cost =
Area * Price_Per_Unit
Relationships may be conditional, e.g.,
For each House_Room:
Area = Length *
Breadth
If House_Room =
Bathroom then
Carpet_Cost = Area * (Price_Per_Unit * 1.2)
Else
Carpet_Cost = Area * Price_Per_Unit
The imperative mindset is to view these:
Area = Length * Breadth
Carpet_Cost = Area *
Price_Per_Unit
as assignment statements -- blocks of computer
memory are to be allocated and modified. The declarative mindset doesn’t
think in those terms at all.
A programmer with a declarative mindset is likely to
create quite different programs than a programmer with an imperative mindset.
RECAP
Declarative programs describe the relationship of
the output to the input.
Declarative programs describe the problem.
Declarative programs define reusable concepts;
functions are typically the programming machinery used for defining concepts.
Declarative programs assemble concepts to create
bigger concepts, i.e., functional composition.
Declarative programs allow the computer to devise
its own execution plan, which may enable the program to be executed in any
order or even in parallel.
Declarative programs do _not_ have instructions or
statements.
Declarative programs do _not_ tell the computer what
to do, i.e., how to solve the problem.
Declarative programs do _not_ tell what values to
put into particular memory locations.
Declarative programs do _not_ have variables that
vary.
Declarative programs do _not_ impose a particular
execution plan on the computer.
/Roger
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