Matching a map exactly

[Scott Thompson]

In writing a function with pattern matching I know I can match a map argument by providing one or more fields:

def my_function(%{direction: some_var})
  # ...do some stuff with some_var ...
end

This function should match maps with extraneous keys like

%{ ignored: "fred", direction: :north}

However, is there a way to mark a matching pattern so that the Map has to match it EXACTLY?  So that the match will not be taken if the Map contains any keys other than the ones specified?

[Bruno Luis Panuto Silva]

Short answer is no. Not in a map.
But on a struct, however, you ensure that you can't put in a field that wasn't defined before.
If you really need maps, you could assert that it matches using a combination of the Map.keys function, like this:

defmodule ExactMap do

   def match(map, keys), do: Map.keys(map) == keys
end

And use it, like:

iex> ExactMap.match(%{ignored: "fred", direction: :north}, [:direction])

[Peter Hamilton]

You can match on all keys and include a guard on map_size.

Bruno's answer about structs isn't quite accurate. There's no protection against structs with extra fields added via Map.put/3. If you always update via the map update syntax %{struct | :key => "value"} then you should be fine, but there nothing but convention protecting you from messing with the strict internals by treating it as a map.

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[Scott Thompson]

This worked perfectly.  Thank you very much.

[Peter Hamilton]

I found another way to do it, thought I'd share here.

iex(1)> z = %{a: 1, b: 2}

%{a: 1, b: 2}

iex(2)> case %{a: 1, b: 2} do

...(2)>   ^z -> true

...(2)>   _ -> false

...(2)> end

true

iex(3)> case %{a: 1, b: 2, c: 3} do

...(3)>   ^z -> true

...(3)>   _ -> false

...(3)> end

false

iex(4)> case %{a: 1, b: 2, c: 3} do

...(4)>   %{a: 1, b: 2} -> true

...(4)>   _ -> false

...(4)> end

true

This won't work in def function heads, but it will work anywhere you can use pinned vars.

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