Common Pitfalls in Elixir Coding

To fall is to raise

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This article is all about common mistakes of newbies in Elixir. Even experts in Elixir some times fall under this category. From the experience, I am revealing some sort of things to be careful while coding. This doesn’t mean that you fall in this category. I am just saying there are pits and you do not fall into it. Don’t worry I am just kidding. Lets jump into virtual pitfalls…

1. Functions with |> operator and parameters

This is the most common pitfall and one has to be more careful while piping the functions with parameters which leads to unusual results. Lets check the following example.

String.graphemes "hello" |> length 

When you execute above line, you will get an ArgumentError . This is because the above lines are grouped as follows

quote(do: String.graphemes "hello" |> length) |> Macro.to_string
"String.graphemes(\"hello\" |> length)"

So, you can use quote and Macro.to_string to see how your coding lines are grouped together. We think in the direction of output of String.graphemes is passed as input to the length function. But it does in other direction of taking the binary "hello" as input to the length function which gives you error as list is expected.

To avoid this we have to use the parenthesis for wrapping the parameters inside them for safe evaluation of code.

String.graphemes("hello") |> length
5

2. Lists Concatenation / improper lists

Elixir provides ++ operator to add elements to the list. Basically it is used for concatenation of two lists, but we always forget to wrap the elements inside []

value = 99
list = [1,2,3,4,5]
list ++ value

We think that the result of list ++ value would be [1,2,3,4,5,99] but in general it will be [1,2,3,4,5|99] . This is a improper list. You cannot use length function over. In proper list, when you iterate over the list, the tail would be [] empty list. This is different with the improper list.

So, to overcome this we have to wrap the value inside the list as
 list ++ [value]

Read more about improper list HERE.

3. Strings with charlists

In Elixir "hello" is not equals to 'hello' . When you use the "" it is considered to be binary in our terms a string and when you use '' , it is a list of characters. This is most common pitfall for newbies. Once if you come to a clear idea on binary and charlists , nothing to fear anymore.

iex> is_binary 'hello'
false
iex> is_list 'hello'
true
iex> is_binary "hello"
true
iex> length 'hello'
5
iex> length "hello"
** (ArgumentError) argument error
:erlang.length("hello")

4. Anonymous function calls and Piping

These are also called as lambda functions which are very handy in data comprehensions. The calling of these functions is little different from regular calling.

square = & &1 * &1

we can write same thing as

square = fn(x)-> x * x end

When you try to access them as square(5) you will get a compile error saying undefined function square/1 . You suppose to call the lambda functions as square.(3) . This is just to differ from the normal functions defined inside the module.

This is very common mistake of newbies in elixir. One has to be very clear with such things. How ever, one can overcome this mistake by using _ in the function name to differ them from the normal functions like _square . So by looking at the name you come to know it as a lambda function.

You can use your own valid sign to differ your lambda functions from the normal functions.

Piping the anonymous functions is also different. You have to do as following

5
|> square.()
or
5
|> (&(&1*&1)).()

5. Boolean value as False which is not false.

In Elixir False is not false . In Elixir only false and nil are considered to be false . The rest all evaluates to true . In C Programming even 0 are considered as false . But in Elixir they are not. In Elixir anything starts with uppercase is also an atom. So, False evaluates to true and surprise us when we think it is false.

So, It is very common to make a mistake with False . Always avoid something with capital letters until or unless it is compulsory.

iex(1)> logic=False
False
iex(2)> if logic do
...(2)> " I am true"
...(2)> else
...(2)> "I am false"
...(2)> end
" I am true"
iex(3)>

6. Guard clause Failures

In elixir we can use the guard clauses in the function definitions by using when . However, if the guard clause has errors, it won’t leak out. It simply fails the guard clause. It will not raise any exception or run time error and looks for another matching definition by considering the previous guard as fail. Lets see the following example

iex> length(15)
** (ArgumentError) argument error
:erlang.length(15)
iex> case 15 do
...> x when length(x) -> "Never match"
...> x -> "value: #{x}"
...> end
"value: 15"

In the above lines of code, when you try to find the length of an integer, It raised an error. But when you used same statement inside the guards it just skipped that definition considering the guard failure. This is very common for newbies making mistake in guard clauses.

7. Keywords order in Function call matters.

In Elixir, the order of passing the keywords as a function parameters really matters. Consider the following lines of code.

defmodule User do
def user name: name,age: age do
"name: #{name}-- age: #{age}"
end
end

Above lines of code defines a module User with function user/1 . While defining the function we asked for name to be first followed by age . Lets check the different ways of calling the above function.

iex> User.user age: 12,name: "hello"
** (FunctionClauseError) no function clause matching in User.user/1
iex:2: User.user([age: 12, name: "hello"])

When you change the order of parameters, you got the no function error raising FunctionClauseError . This is because of Elixir functions are defined along with arity. So, pattern is not matched here.

iex> User.user name: "hello",age: 12
"name: hello-- age: 12"

So, 
def user name: name,age: age 
is not equals to 
def user age: age, name: name

Solution

If you want keywords to be able to passed in any order, you should accept any list in your function head, and then use Keyword.get inside the function instead:

defmodule User do
def user(options) do
name = Keyword.get(options, :user)
age = Keyword.get(options, :age)
"name: #{name}-- age: #{age}"
end
end

This also allows you to specify useful default values for keywords that are not included when the function is called.

This advice is from Wiebe-Marten/Qqwy

8. Retrieving values from Map

In Elixir if you define the map as map = %{name: "hello"} , you can access the map for taking out the name in two ways.

One is using . period like map.name. Beginners in Elixir will always try to access map with . period. It is simple and dangerous too because, it raises an error when you try to extract a key which is not defined in map. Look at the following lines of code.

iex(1)> map = %{name: "hello"}
%{name: "hello"}
iex(2)> map.name
"hello"
iex(3)> map.age
** (KeyError) key :age not found in: %{name: "hello"}

iex(3)> map["name"]
nil
iex(4)> map[:name]
"hello"
iex(5)> map[:age]
nil

In the above lines of code, we are defining a map with name atom as a key. At the line of execution iex(3) , we are trying to take out the value of age using the . period, but it raised an error because, the key is not present.

In the same style, when you try to access the map with access operator [] , it gives you the nil value when the key is not present instead of raising an error. So, it is very handy when you access the map with [:key] or ["key"] To use like map["name"] , the keys in map should be binary as follows

map = %{"name" => "hello"}

This is why when you try with key as binary like map["name"] , it just returned nil because our map is defined with key as atom . So if you try with map[:name] in gives you the value of name .

Note: When you define a map with keys as binary, you cannot access them with . period. It is only accessible when the keys are atoms.

9.Pattern matching limitation

Pattern matching is highly useful and powerful too. How ever you cannot make the functions calls on the left side of the pattern match. Look at the following lines of code.

iex(2)> 3 = length [1,2,3]
3
iex(3)> length [1,2,3] = 3
** (MatchError) no match of right hand side value: 3

In the above lines of code when I try to call the function on the left side to match right side, it raised an error.

Similarly for map datatypes too.

When you try %{}=%{name: "hello"} , this works fine. But, when you reverse the sides like %{name: "hello"}=%{} , this gives you the error. Try and see

iex(3)> %{}=%{name: "hello"}
%{name: "hello"}
iex(4)> %{name: "hello"}=%{}
** (MatchError) no match of right hand side value: %{}

%{name: "hello"}=%{name: "hello",age: 23} it is valid.

%{name: "hello",age: 23}=%{name: "hello"} it is invalid.

So, this is very common to miss understand above terms in pattern matching.

10 Guard clauses using && ,||, ! are invalid

We can check the multiple conditions in the guard clauses. In Elixir you cannot use either && or|| or ! . But coming to guard clauses this is bit different. You have to use and , or, not for && , or and ! respectively. Look at the following example.

iex(13)> x=25
25
iex(14)> case x do
...(14)> y when y>12 && y<30 -> y
...(14)> y -> "nomatch"
...(14)> end
** (CompileError) iex:15: invalid expression in guard
(elixir) expanding macro: Kernel.&&/2
iex:15: (file)

In the above lines of code, we are attempting to check multiple conditions in the guard clause with code when y>12 && y<30 . Even though they look similar, they are not allowed in the guard clauses. If you run the same lines of code by changing && to and, it works fine. Lets check the working example.

iex(14)> case x do               
...(14)> y when y>12 and y<30 -> y
...(14)> y -> "nomatch"
...(14)> end
25

So, there is a chance to make mistake by using &&, or! in guard clause conditions as they work fine in regular condition evaluations . Check the following example.

iex(15)> x=25
iex(16)> if x>12 && x<30 do x else :invalid end
25
iex(17) x=1
iex(18)> if x>12 && x<30 do x else :invalid end
:invalid

Did you observe that&& worked in regular conditions but not allowed in guard clause conditions.

If you find any other pitfall in elixir coding which I did not mention, please share and save others from falling into it.

Love to hear your things. ❤ ❤ ^-^

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