CODEWITHSUNDEEP
elixir
bparikh
bparikh

Reputation: 63

New to Elixir - Concatenate two lists

I'm new to Elixir and I'm trying to create a simple anonymous function that concatenates two lists of atoms.

Correctly, the code written is:

concat = fn(x,y) -> x ++ y end

This code works for other data types as well.

My question is, how come the following code doesn't work? 

list_concat = fn([:a,:b],[:c,:d]) -> [:a,:b,:c,:d] end

Iex doesn't throw when I write in the function, but if I try to run list_concat, the following is thrown:

> list_concat.([:true,:false],[:false,:true])
** (FunctionClauseError) no function clause matching in :erl_eval."-inside 
an-interpreted-fun-"/2

The following arguments were given to :erl_eval."-inside-an-interpreted-fun-
"/2:

    # 1
    [true, false]

    # 2
    [false, true]

Can someone help me figure out what the stack trace means, and why list_concat isn't a correct solution? Is it something to do with pattern matching? Thanks!

Upvotes: 4

Views: 764

Answers (1)

Dogbert
Dogbert

Reputation: 222188

An atom only matches itself. You can only call your list_concat function with [:a,:b] and [:c,:d] as arguments. If you want to bind the variables to any value the user passes, just remove the ::

iex(1)> list_concat = fn [a, b], [c, d] -> [a, b, c, d] end
#Function<12.99386804/2 in :erl_eval.expr/5>
iex(2)> list_concat.([:true, :false], [:false, :true])
[true, false, false, true]

PS: you don't need to put : before boolean values. :true is the same as true.

Upvotes: 3

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