Control Flow

Section

ifs are for 1 or 2 branching paths only.

is_admin = true
owns_blog = true
is_published = true

can_create_blog = is_admin and owns_blog and is_published

# BAD, do not nest ifs.
if is_admin do
  if owns_blog do
    if is_published do
      "CREATE THE BLOG"
    else
      "BLOG MUST BE PUBLISHED"
    end
  else
    "YOU DON'T OWN THIS"
  end
else
  "NOT ADMIN"
end

if can_create_blog do
  "Create the blog"
else
  "Sorry not authorized"
end

Case can handle many branching paths, that work well with pattern matching.

# weather = :sunny
# weather = :rainy
weather = :foggy

case weather do
  :sunny -> "Wear a t-shirt"
  :rainy -> "Wear a raincoat"
  :foggy -> "Wear a long sleave"
end

# names = ["Brooklin"]
names = ["Brooklin", "Drew", "Bill"]

case names do
  [] -> "Where is everyone?"
  ["Drew"] -> "Hello Drew"
  [name] -> "Hello #{name}"
  [name1, name2] -> "Hello #{name1}, Hi #{name2}"
  _names -> "Hello everyone!"
end

# case [1, 2, 3] do
#   true -> "TRUE"
# end

cond is best for multiple branching paths that are not well suited for pattern matching.

grade = 70

# A 86..100
# B 72..85
# C 50..71

cond do
  grade >= 86 and grade <= 100 -> "A"
  grade >= 72 and grade <= 85 -> "B"
  grade >= 50 and grade <= 71 -> "C"
end

# an example where cond is not a great solution.
cond do
  names == [] -> "Where is everyone?"
  length(names) == 1 -> "Hello #{hd(names)}"
  length(names) == 2 -> "Hello #{Enum.at(names, 0)}, Hi #{Enum.at(names, 1)}"
  true -> "Hello everyone!"
end

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