case, cond, and if

In this chapter, we will learn about the case, cond, and if control flow structures.

case

case allows us to compare a value against many patterns until we find a matching one:

iex> case {1, 2, 3} do
...>   {4, 5, 6} ->
...>     "This clause won't match"
...>   {1, x, 3} ->
...>     "This clause will match and bind x to 2 in this clause"
...>   _ ->
...>     "This clause would match any value"
...> end
"This clause will match and bind x to 2 in this clause"

If you want to pattern match against an existing variable, you need to use the ^ operator:

iex> x = 1
1
iex> case 10 do
...>   ^x -> "Won't match"
...>   _  -> "Will match"
...> end
"Will match"

Clauses also allow extra conditions to be specified via guards:

iex> case {1, 2, 3} do
...>   {1, x, 3} when x > 0 ->
...>     "Will match"
...>   _ ->
...>     "Would match, if guard condition were not satisfied"
...> end
"Will match"

The first clause above will only match when x is positive.

Expressions in guard clauses

Elixir imports and allows the following expressions in guards by default:

  • comparison operators (==, !=, ===, !==, >, >=, <, <=)
  • boolean operators (and, or, not)
  • arithmetic operations (+, -, *, /)
  • arithmetic unary operators (+, -)
  • the binary concatenation operator <>
  • the in operator as long as the right side is a range or a list
  • all the following type check functions:
    • is_atom/1
    • is_binary/1
    • is_bitstring/1
    • is_boolean/1
    • is_float/1
    • is_function/1
    • is_function/2
    • is_integer/1
    • is_list/1
    • is_map/1
    • is_nil/1
    • is_number/1
    • is_pid/1
    • is_port/1
    • is_reference/1
    • is_tuple/1
  • plus these functions:
    • abs(number)
    • binary_part(binary, start, length)
    • bit_size(bitstring)
    • byte_size(bitstring)
    • div(integer, integer)
    • elem(tuple, n)
    • hd(list)
    • length(list)
    • map_size(map)
    • node()
    • node(pid | ref | port)
    • rem(integer, integer)
    • round(number)
    • self()
    • tl(list)
    • trunc(number)
    • tuple_size(tuple)

Additionally, users may define their own guards. For example, the Bitwise module defines guards as functions and operators: bnot, ~~~, band, &&&, bor, |||, bxor, ^^^, bsl, <<<, bsr, >>>.

Note that while boolean operators such as and, or, not are allowed in guards, the more general operators &&, ||, and ! are not.

Keep in mind errors in guards do not leak but instead make the guard fail:

iex> hd(1)
** (ArgumentError) argument error
iex> case 1 do
...>   x when hd(x) -> "Won't match"
...>   x -> "Got #{x}"
...> end
"Got 1"

If none of the clauses match, an error is raised:

iex> case :ok do
...>   :error -> "Won't match"
...> end
** (CaseClauseError) no case clause matching: :ok

Note anonymous functions can also have multiple clauses and guards:

iex> f = fn
...>   x, y when x > 0 -> x + y
...>   x, y -> x * y
...> end
#Function<12.71889879/2 in :erl_eval.expr/5>
iex> f.(1, 3)
4
iex> f.(-1, 3)
-3

The number of arguments in each anonymous function clause needs to be the same, otherwise an error is raised.

iex> f2 = fn
...>   x, y when x > 0 -> x + y
...>   x, y, z -> x * y + z
...> end
** (CompileError) iex:1: cannot mix clauses with different arities in function definition

cond

case is useful when you need to match against different values. However, in many circumstances, we want to check different conditions and find the first one that evaluates to true. In such cases, one may use cond:

iex> cond do
...>   2 + 2 == 5 ->
...>     "This will not be true"
...>   2 * 2 == 3 ->
...>     "Nor this"
...>   1 + 1 == 2 ->
...>     "But this will"
...> end
"But this will"

This is equivalent to else if clauses in many imperative languages (although used way less frequently here).

If none of the conditions return true, an error (CondClauseError) is raised. For this reason, it may be necessary to add a final condition, equal to true, which will always match:

iex> cond do
...>   2 + 2 == 5 ->
...>     "This is never true"
...>   2 * 2 == 3 ->
...>     "Nor this"
...>   true ->
...>     "This is always true (equivalent to else)"
...> end
"This is always true (equivalent to else)"

Finally, note cond considers any value besides nil and false to be true:

iex> cond do
...>   hd([1, 2, 3]) ->
...>     "1 is considered as true"
...> end
"1 is considered as true"

if and unless

Besides case and cond, Elixir also provides the macros if/2 and unless/2 which are useful when you need to check for only one condition:

iex> if true do
...>   "This works!"
...> end
"This works!"
iex> unless true do
...>   "This will never be seen"
...> end
nil

If the condition given to if/2 returns false or nil, the body given between do/end is not executed and instead it returns nil. The opposite happens with unless/2.

They also support else blocks:

iex> if nil do
...>   "This won't be seen"
...> else
...>   "This will"
...> end
"This will"

Note: An interesting note regarding if/2 and unless/2 is that they are implemented as macros in the language; they aren’t special language constructs as they would be in many languages. You can check the documentation and the source of if/2 in the Kernel module docs. The Kernel module is also where operators like +/2 and functions like is_function/2 are defined, all automatically imported and available in your code by default.

do/end blocks

At this point, we have learned four control structures: case, cond, if, and unless, and they were all wrapped in do/end blocks. It happens we could also write if as follows:

iex> if true, do: 1 + 2
3

Notice how the example above has a comma between true and do:, that’s because it is using Elixir’s regular syntax where each argument is separated by comma. We say this syntax is using keyword lists. We can pass else using keywords too:

iex> if false, do: :this, else: :that
:that

do/end blocks are a syntactic convenience built on top of the keywords one. That’s why do/end blocks do not require a comma between the previous argument and the block. They are useful exactly because they remove the verbosity when writing blocks of code. These are equivalent:

iex> if true do
...>   a = 1 + 2
...>   a + 10
...> end
13
iex> if true, do: (
...>   a = 1 + 2
...>   a + 10
...> )
13

One thing to keep in mind when using do/end blocks is they are always bound to the outermost function call. For example, the following expression:

iex> is_number if true do
...>  1 + 2
...> end
** (CompileError) undefined function: is_number/2

Would be parsed as:

iex> is_number(if true) do
...>  1 + 2
...> end
** (CompileError) undefined function: is_number/2

which leads to an undefined function error because that invocation passes two arguments, and is_number/2 does not exist. The if true expression is invalid in itself because it needs the block, but since the arity of is_number/2 does not match, Elixir does not even reach its evaluation.

Adding explicit parentheses is enough to bind the block to if:

iex> is_number(if true do
...>  1 + 2
...> end)
true

Keyword lists play an important role in the language and are quite common in many functions and macros. We will explore them a bit more in a future chapter. Now it is time to talk about “Binaries, strings, and char lists”.

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