Matcherror in elixir

Question

I am new to Elixir so I am little confused with these three statements

a = [ [ 1, 2, 3 ] ]

[a] = [ [ 1, 2, 3 ] ]

[[a]] = [ [ 1, 2, 3 ] ]

The first and second statements return result as expected but the third one throws a error

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

I want to know why the third sentence threw an error

Answer 1

a matches any value. [a] matches a list containing exactly one element which can be any value. [[a]] matches a list of exactly one element containing another list of exactly one element which can be any value.

The expression [[1, 2, 3]] matches the first two patterns but doesn't match the third because it's a list of one list containing three elements.

Answer 2

To amplify this even a bit further consider:

a = [[1,2,3]]

# a is [[1,2,3]]

[ a ] = [ [1,2,3] ]
^   ^   ^         ^ These match. 
#a is [1,2,3]

[ [ a ] ] = [ [ 1,2,3 ] ]
^ ^   ^ ^   ^ ^       ^ ^  These match as well. 

# You can consider that the parts that match on the left and the right of 
# the pattern match operator are effectively discarded and then elixir attempts to assign 
# the remaining portion on the right side to the remaining portion on the left side.
# Hence on the last expression it's trying to assign 1,2,3 to a.  The pattern doesn't match 
# because you have three values on the right but only one name on the left to bind to. 

I hope that may help clarify the situation. @Dogbert and @mudasobwa are both completely correct; I just hope this helps to make the situation a little clearer.