每个Erlang模块，如这里应该包含一系列属性和函数声明，每个都由句号（。）终止。

Each Erlang module, as described here, should consist of a sequence of attributes and function declarations, each terminated by period (.)

但这行：

Foo = fun(X) -> X * X end.

...既不是，也应该写成如下：

... is neither and should be written as follows instead:

foo(X) -> X * X.

foo 是小写在这里因为此行是函数声明，其中函数名应为原子码。

foo is lowercase here, because this line is a function declaration, where function name should be an atom.

所以最后你的模块看起来像这样：

So in the end your module will look like this:

-module(bakery).
-export([cake/1]).

foo(X) -> X * X.

cake(0) -> [];
cake(N) when N > 0 -> [ foo(2) | cake(N-1) ].