A Pythonista's Impressions of Erlang
Joe Armstrong (AL/EAB)
Wed Jan 12 14:51:01 CET 2005
> -----Original Message-----
> From: owner-erlang-questions@REDACTED
> Sent: den 12 januari 2005 12:06
> Cc: erlang-questions@REDACTED
> Subject: Re: A Pythonista's Impressions of Erlang
> Speaking of string operations, I need to add (or remove) the
> / at the
> end of an URL if it is absent (or present).
> I only ended in writing ugly code like that:
> list:last(SiteRoot) =/= $/ ->
> SiteRoot ++ [$/];
> true ->
> Is there an elegant way to do last character manipulation?
Actually you're code will crash if SiteRoot is empty - since an empty list
has no last element.
Now you might think that since
"string" ++ Var
Is a legal pattern that you could write your function like this:
add_slash(X = _ ++ "/") -> X;
add_slash(X) -> X ++ "/".
But you would be wrong.
To do it this way involves a quick hack to the compiler
of a rather simple parse_transform to change the semantics of
Actually I see no reason why patterns like Var ++ StringLiteral should not
be compiled, especially since StringLiteral ++ Var *is* allowed - and therefore
disallowing Var ++ StringLiteral violates the principle of least astonishment.
Failing that you'll have to hide the code in a function. Something like this
aardvark() -> "/";
case lists:last(Str) of
$/ -> Str;
_ -> Str ++ "/"
This function is very difficult to name (append_a_slash_at_the_end_of_string_if_there_isnt_one)
isn't a good name, so I've called it aardvark until somebody comes up with a better name
Now there might be a better way. Suppose the resultant string is
just going to be output somewhere.
Then instead of doing Str ++ "/" (which is an expensive operation)
you just build a deep list [Str,"/"] (which is a cheap operation)
and send this to your I/O routines which happily flatten the data as it is output.
Note this programming style is very common -
I write loads of stuff like:
mkList(L) -> ["<ul>",
["<li>", mk_href(I,J) "</li>"]
mkHref(I, J) -> ["<a href='">,I,"'>",J,"</a>"].
happy in the knowledge that these things will eventually get flattened on output.
If you absolutely need a flat version of L just call sneaky_flatten(L)
sneaky_flatten(L) -> binary_to_list(list_to_binary(L)).
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