add pure-functions/index.md (#32887)

A description of the important of pure functions and their limitations in Erlang

guide/english/erlang/pure-functions/index.md

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+---
+title: Pure Functions
+---
+
+## Pure Functions
+
+The concept of 'pure functions' is not difficult to grasp but it is more difficult to grasp just how important
+the concept is to functional programming and how programmers reason about code.
+
+#### More Information:
+<!-- Please add any articles you think might be helpful to read before writing the article -->
+
+  * [Wikipedia: Pure functions](https://en.wikipedia.org/wiki/Pure_function)
+  * [Learn You Some Erlang: Errors and Exceptions](https://learnyousomeerlang.com/errors-and-exceptions)
+
+### What is a Pure Function ?
+
+Think of a subroutine that returns no value at all.
+Well, if it serves any useful purpose, it must do so by side-effects:
+it may change global data, it may be a method changing the state of an object, it may do I/O.
+
+This is technically a (pure) procedure.
+Pure procedures are rare as usually at least a code indicating success / failure is returned.
+
+Consider a subroutine that returns a value and has no side-effects (e.g. square_of/1).
+This is a pure function.
+
+Most programming languages allow a subroutine to be both a procedure with side-effects and a function with a return value.
+
+In functional programming languages, functions are pure (or tend to be) and there are no procedures.
+What are the consequences ?
+
+### Consequences of using Pure Functions
+
+A pure function, given the same input, will always produce the same result.
+This offers compilers more scope for optimisation.
+
+Compilers can examine an entire program to deduce which functions are pure and which are not.
+A programmer cannot do this for a program of any size (even one they wrote themselves).
+
+They can study a subroutine and say "assuming everything else stays the same, it does this" but often checking
+that everything else stays the same is a lot more effort than studying the subroutine.
+
+In a language where all functions are pure, the assumption is guaranteed.
+This simplifies greatly the task of maintaining large applications:
+you can change an individual function with confidence that there will be no unintended side-effects.
+
+### Limitations of a Pragmatic Functional Programming Language
+
+While most functions in Erlang are pure, there are certain features of the language that produce impure functions with side effects:
+
+  * functions may do I/O - I/O has side effects,
+  * actors send messages to one another - see the concurrent half of Erlang,
+  * functions may throw exceptions.
+
+The first two are generally confined to small sections of code so the impurity is not widespread:
+printing out some values for debug or logging purposes in one routine
+does not suddenly mean all the functions around it are under serious threat from side-effects.
+
+Throwing exceptions is a very bad idea.
+It is a double-edged side-effect often there to force some function to clean up another function's run-time mess.
+
+The Erlang philosophy is "Let it crash" so, in general Erlang programmers ignore this side-effect too.
+
+The "Let it crash" philosophy is controversial.
+
+In large, concurrent, Erlang applications, the view is that high reliability is achieved by
+allowing the individual parts of the application to crash and be replaced without stopping the application.
+By failing early, rather than trying to carry on,
+the failed part is less likely to compromise the application or its data.
+Developers can examine the crash and even load a fix without stopping the application.
+
+Throwing an exception becomes a flag that says "If you want to get to the bottom of this bug, dig here".
+
+### How to return errors from functions
+
+Pure functions are all very well but what happens when the arguments to a function means the function cannot determine the result ?
+
+When it is known that this will happen and the caller may reasonably be expected to go to Plan B,
+it makes sense for the function to return a 'return code' as well as the 'return value'.
+
+This can be done using a tuple:
+
+```Erlang
+    % in the called function ...
+
+    if Y /= 0 -> {ok, X / Y};
+       y == 0 -> {err, 0}.
+```
+
+and:
+
+```Erlang
+    % in the caller ...
+
+    case safe_divide(X,Y) of
+        {ok, Result} -> format("That worked");
+        {_, _} -> format("Oops, again");
+    end
+```
+
+<!-- EOF -->