Search for a term or an idea such as pattern matching, exceptions, or short circuiting. Each entry shows its kind, purpose, limits, and a tiny example.
Strictly reserved words appear first. Looking for defmodule, with, or -module? Choose the matching category.
44 entries
Narrow the list by language or category15 strictly reserved words, common special forms, and Kernel macros
afterStrictly reservedCleanup or timeout clausetry ... after runs cleanup before leaving the try block. receive ... after only ends this wait; it does not recall messages that were already sent.
try do
work()
after
cleanup()
endandStrictly reservedStrict Boolean operatorBoth sides must be true or false. If the left side is false, the right side is not run. Use && when you need truthiness with other values.
ready? and enabled?catchStrictly reservedCatch clauseOrdinary exceptions usually belong in rescue. catch mainly handles throw/1, process exits, and lower-level error classes.
try do
throw(:stop)
catch
:throw, reason -> reason
enddoStrictly reservedStart of a code blockWrite several lines as do ... end. For one expression, you can use the shorter do: expression form.
if ready?, do: run()elseStrictly reservedAlternative clauseIt can appear in if, unless, with, and try. Its trigger is slightly different in each construct.
if ok?, do: :yes, else: :noendStrictly reservedEnd of a code blockIndentation helps people read the code, but it is not syntax. A multiline block must end with end.
if ready? do
run()
endfalseStrictly reservedBoolean literalElixir treats only false and nil as falsy. Zero, an empty string, and an empty list are all truthy.
active? = falsefnStrictly reservedAnonymous functionAn anonymous function may have several pattern-matching clauses. After storing it in a variable, call it with the dot form: variable.(arguments).
double = fn x -> x * 2 end
double.(3)inStrictly reservedMembership operatorin is also allowed in guards. It only tests membership; comprehensions use <- to draw values from a collection.
2 in 1..3nilStrictly reservedEmpty-value literalnil is the literal form of the atom :nil and is falsy in conditions. It often means “not found” or that an optional value is missing.
result = nilnotStrictly reservedStrict Boolean operatornot accepts only true or false. Use ! when you need to invert the truthiness of nil or other values.
not finished?orStrictly reservedStrict Boolean operatorBoth sides must be true or false. If the left side is true, the right side is not run. Use || for truthiness with other values.
cached? or fetch?()rescueStrictly reservedException clauseYou can catch only named exceptions such as ArgumentError. An exception that does not match keeps travelling outward.
try do
risky()
rescue
ArgumentError -> :invalid
endtrueStrictly reservedBoolean literaltrue is the literal form of the atom :true. It is also often the final cond branch, meaning “all remaining cases.”
enabled? = truewhenStrictly reservedGuard clauseA guard after when may use only approved operators and functions. You cannot place any ordinary function there.
def positive?(n) when n > 0, do: true29 strictly reserved words, common module attributes, and preprocessor directives
afterStrictly reservedTimeout or cleanup clausereceive ... after only stops the wait when time runs out. Expressions in try ... after run before the try expression is left.
receive
Msg -> Msg
after 1000 ->
timeout
endandStrictly reservedStrict Boolean operatorBoth sides must be Boolean values, and the right side always runs. Use andalso when the right side should not run after a false left side.
true and false. % falseandalsoStrictly reservedShort-circuit Boolean operatorBoth sides should produce Boolean values. Put a safety check first to avoid an unnecessary or unsafe calculation on the right.
N > 0 andalso N rem 2 =:= 0.bandStrictly reservedBitwise operatorIt works only on integers. It is often used to read masks, flags, or selected bits in a binary protocol.
6 band 3. % 2beginStrictly reservedExpression blockSeparate the steps with commas. They run in order, and the whole begin ... end expression returns the value of the last step.
begin A = 1, A + 1 end.bnotStrictly reservedBitwise operatorErlang integers have no fixed 8-bit or 32-bit limit, so the result follows arbitrary-precision integer rules.
bnot 0. % -1borStrictly reservedBitwise operatorIt works only on integers and is often used to combine independent bit flags.
4 bor 1. % 5bslStrictly reservedBit-shift operatorWith a positive shift count, this usually acts like multiplication by the matching power of 2. It accepts only integers.
3 bsl 2. % 12bsrStrictly reservedBit-shift operatorIt accepts only integers. Negative integers use an arithmetic right shift, preserving their sign meaning.
12 bsr 2. % 3bxorStrictly reservedBitwise operatorMatching bits produce 0 and different bits produce 1. This is an integer bit operation, not Boolean xor.
6 bxor 3. % 5caseStrictly reservedPattern branchcase evaluates its target once, then tries the patterns and optional guards after of from top to bottom. It raises an error if nothing matches.
case X of
0 -> zero;
_ -> other
end.catchStrictly reservedException captureIt appears in the older catch Expression form and in a try catch clause. New code usually prefers the clearer structure of try.
Result = catch risky().condStrictly reservedReserved but unusedcond cannot be used directly as an atom, variable, or function name. Write 'cond' when you need that atom, and use case or if for branches.
erl_scan:reserved_word('cond'). % truedivStrictly reservedArithmetic operatorThe result is truncated toward zero, not rounded. A zero divisor raises a badarith error.
7 div 2. % 3elseStrictly reservedmaybe failure branchThe clauses after else still choose by pattern. This belongs only to maybe syntax; it is not a general if-else form.
maybe {ok, V} ?= fetch(), V else error -> missing end.endStrictly reservedEnd of an expression blockIndentation cannot close these expressions. Every opened multiline block needs an explicit end.
fun(X) -> X * 2 end.funStrictly reservedAnonymous function or function referenceAn anonymous function may have several pattern clauses. fun Module:Function/Arity refers to an existing remote function.
Double = fun(X) -> X * 2 end.ifStrictly reservedGuard branchesAn if condition must be an allowed guard expression, not any function call. If no branch matches, Erlang raises an if_clause error.
if N > 0 -> positive; true -> other end.letStrictly reservedReserved but unusedErlang binds variables through pattern matching. If you truly need an atom named let, write it as 'let'.
erl_scan:reserved_word('let'). % truemaybeStrictly reservedSequential matching expression?= is used only inside maybe. A failed match returns the failing value, or passes it to else for a common result.
maybe
{ok, V} ?= fetch(),
V
end.notStrictly reservedBoolean operatornot accepts only Boolean values. Passing a number, atom, or another kind of value raises a badarg error.
not false. % trueofStrictly reservedPattern-clause markercase uses of to introduce its branches. of is optional in try, where it handles only values returned normally by the try body.
case Value of ok -> done; _ -> retry end.orStrictly reservedStrict Boolean operatorBoth sides must be Boolean values, and the right side always runs. Use orelse when the right side should not run after a true left side.
false or true. % trueorelseStrictly reservedShort-circuit Boolean operatorBoth sides should produce Boolean values. When the left side is already true, the right side does not run, so it works well for fallback conditions.
Cached orelse filelib:is_file(Path).receiveStrictly reservedProcess message receptionMessages are scanned in arrival order, and unmatched messages stay in the mailbox. Add after to avoid waiting forever.
receive
{ping, From} -> From ! pong
end.remStrictly reservedArithmetic operatorrem pairs with div and follows X = (X div Y) * Y + (X rem Y). A zero divisor raises an error.
7 rem 2. % 1tryStrictly reservedException-handling expressiontry can combine of, catch, and after to handle normal values plus error, exit, and throw. An unmatched error keeps travelling outward.
try risky() catch _:Reason -> {error, Reason} end.whenStrictly reservedGuard clauseOnly approved guard expressions may follow when. Ordinary functions cannot be placed there freely; a false condition moves on to the next clause.
abs(N) when N < 0 -> -N.xorStrictly reservedBoolean operatorBoth sides run and both must be Boolean values. Use bxor for an exclusive-or operation on every bit of two integers.
true xor false. % trueReserved words, special forms, macros, and module attributes follow different rules. Learn the job first, then remember the category.
The language already uses these names, so an ordinary variable or function cannot reuse them.
Elixir: when · fn · do
Erlang: case · receive · funThey form Elixir's basic syntax and are handled directly by the compiler, but most are not strictly reserved words.
case · with · receive
alias · quote · unquoteThey look like keywords, but may really be macros, module attributes, or preprocessor directives.
defmodule · use · if
-module · -spec · -defineThis page follows the official references. Erlang maybe was disabled by default in OTP 25–26 and enabled by default from OTP 27. cond and let are reserved but still have no active grammar role.