As Jan says in the hierarchically-above page (which took me some time to click on):

For example (note that \n is a newline):

?- normalize_space(atom(Out), '  \nhello\nworld ').
Out = 'hello world'.

So this predicate takes a tagged value to specify the output type (its a special Prolog way of doing things; I would be looking for an options list (which I consider better because you can pass several different flags) or a differently named predicate (which I consider worse because the space of predicates becomes mentally messy))

?- normalize_space(string(X),hello).     % atom hello to string
X = "hello".

?- normalize_space(atom(X),hello).       % atom hello to atom
X = hello.

?- normalize_space(atom(X)," hello ").   % string to trimmed atom
X = hello.

?- normalize_space(string(X)," hello "). % string to trimmed string
X = "hello".

?- normalize_space(string(y)," hello "). % the result is not the atom 'y'
false.

?- normalize_space(string(hello)," hello "). % the result is not the atom 'hello'
false.

?- normalize_space(string("hello")," hello "). % the result is indeed the string "hello"
true.

?- normalize_space(atom("hello")," hello "). % the result is not the string "hello"
false.

?- normalize_space(atom(X)," hello    world  ").  % inner space is normalized
X = 'hello world'.

?- normalize_space(123," hello    world  "). % just throws
ERROR: Type error: `output' expected, found `123' (an integer)

?- normalize_space(atom(hello),X). % throwing as a way of saying "you are going too far"
ERROR: Arguments are not sufficiently instantiated

Should the documentation say that In can only be an atom or a string (not a list of codes or chars)?