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I have been trying to learn Erlang and have been running into some problems with ending lines in functions and case statements.
When do I use a semicolon (;), comma (,), or period inside my functions or case statements?
935
Semicolon at the end of case statement, or if statement, etc. The last case or if statement doesn't have anything at the end. A period at the end of a function. You also use the semicolon at the end of a function clause which doesn't end the function (you end a function by using a dot, as marcc explained).
Use semicolons to replace commas when individual items in a series are long or contain commas. Use a semicolon instead of a comma to separate the items.
A semicolon is most commonly used to link (in a single sentence) two independent clauses that are closely related in thought. When a semicolon is used to join two or more ideas (parts) in a sentence, those ideas are then given equal position or rank.
In Erlang, all the variables are bound with the '=' statement. All variables need to start with the upper case character. In other programming languages, the '=' sign is used for the assignment, but not in the case of Erlang.
I like to read semicolon as OR, comma as AND, full stop as END. So
foo(X) when X > 0; X < 7 ->
Y = X * 2,
case Y of
12 -> bar;
_ -> ook
end;
foo(0) -> zero.
reads as
foo(X) when X > 0 *OR* X < 7 ->
Y = X * 2 *AND*
case Y of
12 -> bar *OR*
_ -> ok
end *OR*
foo(0) -> zero *END*
This should make it clear why there is no ; after the last clause of a case.
130
Comma at the end of a line of normal code.
Semicolon at the end of case statement, or if statement, etc.
The last case or if statement doesn't have anything at the end.
A period at the end of a function.
example (sorry for the random variable names, clearly this doesn't do anything, but illustrates a point):
case Something of
ok ->
R = 1, %% comma, end of a line inside a case
T = 2; %% semi colon, end of a case, but not the end of the last
error ->
P = 1, %% comma, end of a line inside a case
M = 2 %% nothing, end of the last case
end. %% period, assuming this is the end of the function, comma if not the end of the function
In modules, the period is used to terminate module attributes and function declarations (a.k.a. 'forms'). You can remember this because forms aren't expressions (no value is returned from them), and therefore the period represents the end of a statement.
Keep in mind that definitions of functions with different arities are considered separate statements, so each would be terminated by a period.
For example, the function definitions for hello/0 and hello/1:
hello() -> hello_world.
hello(Greeting) -> Greeting.
(Note that in the erlang shell the period is used to terminate and evaluate expressions, but that is an anomaly.)
The semicolon acts as a clause separator, both for function clauses and expression branches.
Example 1, function clauses:
factorial(0) -> 1;
factorial(N) -> N * fac(N-1).
Example 2, expression branches:
if X < 0 -> negative;
X > 0 -> positive;
X == 0 -> zero
end
The comma is an expression separator. If a comma follows an expression, it means there's another expression after it in the clause.
hello(Greeting, Name) ->
FullGreeting = Greeting ++ ", " ++ Name,
FullGreeting.
You can think of it like english punctuation. Commas are used to separate things in a series, semicolons are used to separate two very closely related independent clauses[1] (e.g. the different cases of the case statement, function clauses of the same name and arity that match different patterns), and periods are used to end a sentence (complete thought).
40
The comma separates expressions, or arguments, or elements of a list/tuple or binary. It is overworked.
26
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