How do I compare empty string with ""?

Question

I want to understand why

(when (string= issue "")
    (error "Issue number missing"))

doesn't throw an error when issue is empty, but

(when (= (length issue) 0)
    (error "Issue number missing"))

works as expected.

Documentation of string= states that it compares content of both strings and returns t when they are identical. I'm pretty sure a string of length 0 and "" have identical contents.

The value of issue is set in the following code:

(string-match "\#\\([0-9]*\\)" text)
(setq issue (match-string 1 text))

I tried to replace when with if and added (print (concat "\"" issue "\"")) to the else clause, it printed "\"\"" as a result. I printed the POST query that uses issue as one of its inputs and it generated empty string in its place (the code constructing the query shouldn't even execute if string= returned the correct value). Not to mention that it's length is equal to 0. I'm certain that issue is equal to "".

(let ((issue "")) (string= issue "")) returns t for me. Are you sure issue is an empty string? — choroba
– choroba, Commented Feb 2, 2023 at 15:26
If the match fails, then issue is set to nil, not to "". — NickD
– NickD, Commented Feb 2, 2023 at 15:59
concat turns nil into "", try printing it without concat, too. — choroba
– choroba, Commented Feb 2, 2023 at 16:06

choroba · Accepted Answer · 2023-02-02 16:09:08Z

2

Empty string compares correctly to an empty string:

(let ((issue "")) (string= issue "")) 
-> t

issue is nil, not empty.

(string= nil "")
-> nil

The test is misleading because concat stringifies nil to an empty string:

(print nil)
-> nil
(print (concat nil))
-> ""

answered Feb 2, 2023 at 16:09

choroba

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