Re: checking for nil argument in interactive

[Tassilo Horn <tassilo@member.fsf.org>]

Seb <spluque@gmail.com> writes:

Hi Seb,

> I don't understand why the following arg symbol doesn't get set to
> "string":
>
>
> (defun funny-function (arg)
>   (interactive "sarg: ")
>   (if (not arg) (setq arg "string"))
>   (message "arg: %s" arg))
>
> To test:
>
> M-x funny-function RET RET

Because in that case arg is the empty string.

> I've also tried (eq "" arg) for the if boolean but get the same
> result.

Because (eq "foo" "foo") ==> nil in the general case.

See the manual for details.

,----[ (info "(elisp)Equality Predicates") ]
|  -- Function: eq object1 object2
|      This function returns `t' if OBJECT1 and OBJECT2 are the same
|      object, `nil' otherwise.
| 
|      `eq' returns `t' if OBJECT1 and OBJECT2 are integers with the same
|      value.  Also, since symbol names are normally unique, if the
|      arguments are symbols with the same name, they are `eq'.  For
|      other types (e.g., lists, vectors, strings), two arguments with
|      the same contents or elements are not necessarily `eq' to each
|      other: they are `eq' only if they are the same object, meaning
|      that a change in the contents of one will be reflected by the same
|      change in the contents of the other.
| 
|           (eq 'foo 'foo)
|                => t
| 
|           (eq 456 456)
|                => t
| 
|           (eq "asdf" "asdf")
|                => nil
`----

So to compare stings use `string='.

Hope that helps,
Tassilo
-- 
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