How to pattern match head in a list in function?-CodePudding

I have a list of mix of integer and atom. I want to match the head of the list with atom otherwise integer.

lst = [1,2,3,4,5,6, :eoe, 7,8,9,10,11,12. :eoe]

I initially tried this way:


defmodule Test do
  def test(lst) do
    helper(lst, 0, 0, 1)
  end

  def helper([], _p, total, e) do
    IO.puts "#{e} #{t}"
  end

  def helper([:eoe , t], _p, total, e) do   # <--- This function never called even though head is at some point :eoe
    IO.puts "#{e} #{total}"

    helper(t, "", 0, elf   1)
  end

  def helper([h | t], p, total, e) do
    h
    |> is_atom()
    |> IO.inspect()

    helper(t, h, total   h, e)

  end
end

then added guards to explicitly narrow down pattern matching

...

def helper([:eoe = head , t], _p, total, e) when is_atom(head) do
...

def helper([h | t], p, total, e) when is_integer(h) do
...

def helper([:eoe = h , t], _p, total, e) when is_atom(h) do this function does't get called. It always matches def helper([h | t], p, total, e) when is_integer(h) do this one. I even placed the former one before latter one. I would expect it be matched against :eoe

CodePudding user response：

To match the head one should use the [h | t] notation. [h, t] would match the list of two elements.

- def helper([:eoe = h, t]
  def helper([:eoe = h | t]

Also, when is_atom(h) guard is redundant, once you pattern-match directly on the atom. That said, any of the following would do.

def helper([:eoe | t], _p, total, e) do
def helper([h | t], _p, total, e) when h == :eoe do
def helper([h | t], _p, total, e) when h in [:eoe] do
def helper([h | t], _p, total, e) when is_atom(h) do # match any atom