记录模式

Question

I am learning erlang and I stumbles over some behaviour I cannot quite understand. Take this piece of code. (I know there are existing libraries for what I am programming, but as I stated, I do this for educational purposes):

-module (codec).
-compile (export_all).
-record (node, {symbol, weight, order, left, right, parent} ).
-record (tree, {root, nodes} ).

highestOrderForWeight (Weight, Tree) ->
    lists:max ( [Node#node.order || Node <- Tree#tree.nodes, Node#node.weight == Weight] ).

swapMaybe (Node, Tree) ->
    case highestOrderForWeight (Node#node.weight, Tree) of
        Node#node.order -> pass; 
        Node#node.parent -> pass;
        Tree#tree.root -> pass;
        Partner -> io:format ("Swapping ~p with ~p.~n", [Node#node.order, Partner] )
    end.

The compiler is not at all amused about my code:

./so.erl:11: illegal pattern
./so.erl:12: illegal pattern
./so.erl:13: illegal pattern
error

It has appearently some trouble digesting records in patterns, because when I change my code to this clumsy work-around, it compiles fine:

swapMaybe2 (Node, Tree) ->
    [Order, Parent, Root] = [Node#node.order, Node#node.parent, Tree#tree.root],
    case highestOrderForWeight (Node#node.weight, Tree) of
        Order -> pass; 
        Parent -> pass;
        Root -> pass;
        Partner -> io:format ("Swapping ~p with ~p.~n", [Node#node.order, Partner] )
    end.

Questions:

• How do I access record fields in patterns?
• If it is not possible to do so, why is that so?
• If it is not possible to do so, what is the common practice to work around that?

Answer 1

Actually records are just a compile time syntactic sugar and you can look at the actual constructs by using 'E' compiler option . For example Node#node.order will be replaced by something like this:

case Node of
    {node,_,_rec0,_,_,_} ->
        rec0;
    _ ->
        error({badrecord,node})
end

And of course when you try to use Node#node.order as a patter compiler reports illegal pattern for this construct.

Your swapMaybe function can be rewritten like this:

swapMaybe(#node{order=Order, parent=Parent}, Tree=#tree{root=Root}) ->
    case highestOrderForWeight (Weight, Tree) of
        Order -> pass; 
        Parent -> pass;
        Root -> pass;
        Partner -> io:format ("Swapping ~p with ~p.~n", [Order, Partner] )
    end.

Answer 2

It's indeed not possible to use records in case statements the way you did. Pattern matching records works like this:

 swapMayBe2(#node{order=Order, parent=Parent, root=Root} = Node, Tree) ->
     ...

This binds Order to the field order etc.

Take a look at the Erlang Programming Examples User's Guide: http://www.erlang.org/doc/programming_examples/records.html#id62786

Answer 3

A pattern is not an arbitrary expression that evaluates to the thing you want to match against - you can't for example write:

  case ... of
      1 + 2 -> ...

and your attempt to match against the value of a field of a record:

  case some_integer(...) of
      Node#node.order -> ...

is really just the same kind of thing. A pattern always has the form of a constructor - it describes the shape of a thing, not how it is computed. As you noted, pre-instantiated variables can be used:

  Order = Node#node.order,
  case some_integer(...) of
      Order -> ...

The more common solution is to put the computed value in a guard, if the expression you want is so simple that it's allowed in guards:

case some_integer(...) of
    Value when Value =:= Node#node.order -> ...

If the expressions are short, you might want to combine them in a single clause, using a semicolon as separator in the guard:

case some_integer(...) of
    V when V =:= Node#node.order ; V =:= Node#node.parent ; V =:= Node#node.root ->
        ...;
    Other ->
        ...
end

(Finally, as a matter of style, please don't put a space between the function name and the opening parenthesis of the argument list.)