deckset.md

footer: © Peter Mueller, 2019 slidenumbers: true

What is Pattern Matching

and How to Use the Debugger

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Peter Mueller

• @felix_starman
• felix-starman
• slides at: gh/felix-starman/what_is_pattern_(...)

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Pattern Matching

First let's talk about "assignment"

// javascript
let some_var = ["menuItem", 37, "Crunchy Frog"]

# ruby
some_var = [:menu_item, 37, "Crunchy Frog"]

# elixir
some_var = [:menu_item, 37, "Crunchy Frog"]

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Pattern Matching

There is even multiple-assignment in some languages

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// javascript: destructuring assignment
[type, id, title] = ["menuItem", 37, "Crunchy Frog"]

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# ruby: multiple-assignment
[type, id, title]  = [:menu_item, 37, "Crunchy Frog"]

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# elixir: matching "assignment"
[type, id, title]  = [:menu_item, 37, "Crunchy Frog"]

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Pattern Matching

But "Matching" is not quite assignment

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# elixir
fav_fruit = %{apples: ["Honey Crisp", "Granny Smith"]}
%{apples: apples} = fav_fruit
apples # ["Honey Crisp", "Granny Smith"]

%{durian: true} = fav_fruit
# (MatchError) no match of right hand side value: %{apples: ["Honey Crisp", "Granny Smith"]}

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Pattern Matching

You can match on the shape, but assign nothing

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# Maps
%{username: _} = user_session
%{username: _, admin: true} = user_session
%{username: "demosthenes"} = user_session

%{author: ^current_user} = article

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Matching on other data structures

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# Strings
"NAME=" <> name = commandline_args # String matching

# Lists
[un, pw] = String.split("my_user:password1", ":")

# Tuples
{:ok, result} = VeryImportantThing.do_it()

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Matching in function definitions

Little ugly

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def authorized?(conn) do
  if conn.admin do
    true
  else
    if conn.current_user == conn.owner_of_thing do
      true
    else
      false
    end
  end
end

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Matching in function definitions

Awwwwwww yeaaaah

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def authorized?(%{admin: true}), do: true
def authorized?(%{owner: user, current_user: user}), do: true
def authorized?(_), do: false

# These are "definitions" for ONE function (`authorized?/1`)
# Patterns are evaluated in the order they are defined

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Debugger