How can I compare a variable that is passed to a function with a string in CLIPS?

When I tried to compare a variable with a string, it gives me an error. I tried to compare it with (= ?a "s").

This is the full code example that produces the error:

(deffunction cierto (?a)
  (if (= ?a "s")
    then
      (printout t TRUE crlf)
    else
      (printout t FALSE crlf)
  )
)

The error:

Defining deffunction: cierto
[ARGACCES5] Function = expected argument #2 to be of type integer or float

ERROR:
(deffunction MAIN::cierto
   (?a)
   (if (= ?a "s")
FALSE

Solution

(deffunction cierto (?a)
  (if (eq ?a "s")
    then
      (printout t TRUE crlf)
    else
      (printout t FALSE crlf)
  )
)

(= ) is for comparing numbers (INTEGER or FLOAT) for equality.

(= 3 3.0) is TRUE
(= 3 3) is TRUE
(= s s) ERROR, s is not a NUMBER

(eq ) is for comparison of PRIMITIVE values and also comparison of types)

(eq 6 6.0) is FALSE, different types (INTEGER vs. FLOAT)
(eq 6 6) is TRUE, same type same value (INTEGER INTEGER)
(eq si "si") is FALSE, different types (SYMBOL vs. STRING)
(eq si si) is TRUE, same type, same values (SYMBOL SYMBOL)
(eq "si" "si") is TRUE, same type same values (STRING STRING)

More information in the Basic Programming Guide (Section 12: Actions And Functions)

Alternative:

(deffunction cierto2 (?a)
  (printout t (eq ?a "s") crlf)
)

You cold also use a SYMBOL s instead of a STRING "s".