postgresql- how to define a variable by a function call in an if statement [closed]

I have this function:

CREATE OR REPLACE FUNCTION findMyList( p_user_id bigint)
 .... 

DECLARE

   myVar_id bigint;

...

      IF myVar_id = select p_user_id from myFunction(p_user_id )  THEN
       NULL;
     ELSE
       myVar_id := NULL;
     END IF;

however there appears to be a syntax issue that I can’t work out why it does not allow this as it doesnt seem unreasonable:

ERROR:  syntax error at or near "select"
LINE 78:      IF myVar_id = select p_user_id from myFunction...

why does it not let me define the variable as such?

Answer

Try :

myVar_id := (SELECT p_user_id from myFunction(p_user_id))