Four Js Development Tools Forum

I'm busy looking into implementing the new Genero Methods into an application and was wondering how to return an Interface from a Function

I have two example Types set up as

TYPE TypeA RECORD
    notUsed STRING
END RECORD
 
FUNCTION (typeA TypeA) test()
    DISPLAY "Is TypeA"
END FUNCTION
 

And

TYPE TypeB RECORD
    notUsed STRING
END RECORD
 
FUNCTION (typeB TypeB) test()
    DISPLAY "Is TypeB"
END FUNCTION
 

Then an Interface to use them with as

TYPE IType INTERFACE
    test()
END INTERFACE
 

It seems to work fine but how can they be used in a scenario like the below where I need to return an Interface depending on an argument

MAIN
    DEFINE typeA TypeA
 
    CALL typeBuilder("typeA") RETURNING typeA.*
    CALL polyTest(typeA)
END MAIN
 
FUNCTION typeBuilder(request STRING)
    DEFINE typeA TypeA, typeB TypeB
 
    IF request = "typeA" THEN
        RETURN typeA.*
    ELSE
        RETURN typeB.*
    END IF
END FUNCTION
 
FUNCTION polyTest(itype IType)
    CALL itype.test()
END FUNCTION
 

I'm trying to use in the code in MAIN as

MAIN
    DEFINE itype IType
 
    CALL typeBuilder("typeA") RETURNING itype
    CALL polyTest(itype)
END MAIN
 

Hi Trent, you need to make use of the optional RETURNS clause of the FUNCTION typeBuilder
Also , since 4.1.x you don't need to return record.* anymore.
Just do this

FUNCTION typeBuilder(request STRING) RETURNS IType
   DEFINE typeA TypeA, typeB TypeB
 
   IF request = "typeA" THEN
       RETURN typeA
   ELSE
       RETURN typeB
   END IF
END FUNCTION
 

Regards , Leo

Hello Leo, thanks so much for the response.

I've updated the code and it is compiling fine which is great but I am getting a "Null pointer exception" on the CALL itype.test() line in the polyTest FUNCTION.

Do I need to make any other changes for it to work with Genero 3.20? (I should have mentioned I'm not using 4.10 yet)

Hi Trent , I needed to fiddle a bit to make it working in 3.20,
the following code should compile and run at your side,
see the comments which explain a bit the difference to 4.x
Of course this code works also once you switch to 4.x

TYPE TypeA RECORD
   notUsed STRING
END RECORD
 
TYPE TypeB RECORD
   notUsed STRING
END RECORD
 
TYPE IType INTERFACE
   test()
END INTERFACE
 
--need module scope vars for the type builder in 3.20
--and var names shouldn't be type names
--(genero is still case insensitive)
--DEFINE typeA TypeA won't work in 3.20
--also type and var definitions can't be freely mixed
--with function as its done in 4.x
DEFINE tA TypeA
DEFINE tB TypeB
 
FUNCTION typeBuilder(request STRING) RETURNS IType
   DEFINE it IType
 
   IF request = "typeA" THEN
       LET it=tA
   ELSE
       LET it=tB
   END IF
   RETURN it
END FUNCTION
 
FUNCTION (typeA TypeA) test()
   DISPLAY "Is TypeA"
END FUNCTION
 
FUNCTION (typeB TypeB) test()
   DISPLAY "Is TypeB"
END FUNCTION
 

Regards, Leo

Trent, I double checked against 4.x: also in 4.x you can't return an interface variable (which is a kinda pointer) from a variable which has function local scope.
The runtime does set it to NULL on return because the local scope of the typeBuilder was  left ...and there is no mechanism in Genero which allows to point to scopes which have been destroyed.
(It would be possible in theory by creating a temp variable and using a garbage collector).
So just use variables which have module scope for the builder and you are fine.
Regards, Leo

Hello Leo, oh ok I see, a bit unexpected but I think I can work with this. Thanks so much for your help! I would never have figured that out, haha