Illegal expression when pattern matching with maps in Erlang

Question

I'm trying to execute sample code on the interactive shell from Armstrong's Erlang book. This is what the book says is the case:

1> Henry8 = #{ class => king, born => 1491, died => 1547 }. #{ born => 
1491, class=> king, died => 1547 }.
2> #{ born => B } = Henry8.
#{ born => 1491, class=> king, died => 1547 }.

However, this is what I'm getting on the shell, it seems the pattern matching is failing:

1> Henry8 = #{ class => king, born => 1491, died => 1547 }.
#{born => 1491,class => king,died => 1547}
2> #{ born => B } = Henry8.
* 1: illegal pattern

Answer 1

=> is for constructing a map. To pattern match a map, you need to use := instead.

1> Henry8 = #{ class => king, born => 1491, died => 1547 }.
#{born => 1491,class => king,died => 1547}
2> #{ born := B } = Henry8.
#{born => 1491,class => king,died => 1547}
3> B.
1491

This is documented in the section "Maps in Patterns" here.

Answer 2

The code example was preceded by the text:

Pattern Matching the Fields of a Map
The := syntax we used in a map literal can also be used as a map pattern.

And that text was preceded by a whole section explaining the differences between => and := when constructing a map, so you should have been aware of the two different syntaxes.

In the book, line 2 of the example says:

2> #{born := B} = Henry8.

yet in the shell you typed:

2> #{ born => B } = Henry8.

I suggest you reread section 5.3 a little more carefully, and also read the pertinent section of LYSE, which includes this example:

1> Pets = #{"dog" => "winston", "fish" => "mrs.blub"}.
#{"dog" => "winston","fish" => "mrs.blub"}

2> #{"fish" := CatName, "dog" := DogName} = Pets.
#{"dog" => "winston","fish" => "mrs.blub"}

7> CatName.
"mrs.blub"

8> DogName.
"winston"

Here it's possible to grab the contents of any number of items at a time, regardless of order of keys. You'll note that elements are set with => and matched with :=. The := operator can also be used to update an existing key in a map