string-trim

Why does (string-trim '(#\Tab) nil) return "NIL" and not nil?

On 2008-04-02 21:14:37 -0400, wrf3@stablecross.com (Bob Felts) said:

> Why does (string-trim '(#\Tab) nil) return "NIL" and not nil?

Because string-trim returns a string not a string designator. "NIL" is
a string and nil, though a string designator and therfore suitable as
the second arg, is not a string and so cannot be the return value.

"Function STRING-TRIM, STRING-LEFT-TRIM, STRING-RIGHT-TRIM

Syntax:
string-trim character-bag string => trimmed-string
string-left-trim character-bag string => trimmed-string
string-right-trim character-bag string => trimmed-string


Arguments and Values:

character-bag---a sequence containing characters.
string---a string designator.
trimmed-string---a string."

wrf3@stablecross.com (Bob Felts) writes:

> Why does (string-trim '(#\Tab) nil) return "NIL" and not nil?

Because symbols are "string designators" which can be used in place of
strings.  And nil is a symbol with the symbol-name "NIL", which is
therefore used as the string being processed by STRING-TRIM.

Compare:

(string-trim '(#\Tab) "NIL")
(string-trim '(#\Tab) 'foo)
(string-trim '(#\Tab) t)


--
Thomas A. Russ,  USC/Information Sciences Institute

På Thu, 03 Apr 2008 03:14:37 +0200, skrev Bob Felts <wrf3@stablecross.com>:

> Why does (string-trim '(#\Tab) nil) return "NIL" and not nil?

As a curiosity string-lessp treats nil as "NIL" as well.
(At least under LispWorks.)

--------------
John Thingstad