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In MapBasic it seems like Functions can have side effects so I don't really see the difference between using the following:

Declare Sub MyProcedure (myParameter As String) and then use myParameter after Call MyProcedure(myParameter)


Declare Function MyProcedure () As string and do something like myParameter = MyProcedure()

Am I missing something?

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What side-effects? Otherwise yes, the 2 setups do the same. But using a function call is cleaner imho, in this case. – Uffe Kousgaard Feb 2 '14 at 13:27
@UffeKousgaard a MapBasic function can modify state (of a global variable for instance) other than the return value. As described on wikipedia on side effects – steenhulthin Feb 2 '14 at 13:32
OK, but so can Sub's. Global variables are truly global in Mapbasic. Not very modern in any sense. – Uffe Kousgaard Feb 2 '14 at 13:37
up vote 7 down vote accepted

A Function always returns a value where as a Sub does not. All methods in MAPBASIC can have side effects in the true term because it's not a functional language.

If you are expecting a return value you should use a Function that is what they are there for.

Can you modify the value of myParameter inside a Sub and then just use it after. Yes. But this is messy and generally not good programming

Sometimes modifying a input variable is a way to have multiple outputs in languages that support multiple return types (in something like Python you can return as many things as you want)

An example might be:

Declare Function Convert(passed as Boolean) As string

Function Convert(passed as Boolean) As string
   passed = True
   return "Converted"
End Function

Then you can check passed after running the function.

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@peter-horsboll-moller might have to check my mapbasic knowledge there ;) – Nathan W Feb 2 '14 at 13:04
You passed, Nathan. Have you been reading your MapBasic reference guide again lately? :-) – Peter Horsbøll Møller Feb 2 '14 at 18:45
And not that if you don't want your procedure to pass back a modified paramter, you can use the ByVal keyword – Peter Horsbøll Møller Feb 2 '14 at 18:46

Reading a bit more up on MapBasic I can see that one difference is that it is not possible to pause execution of a Function with Stop according to the MapBasic user Guide. But this restriction does not apply to Subs.

share|improve this answer
Well, yeah. You are touching some of the not so good stuff in the MapBasic language - Debugging. I have been coding MapBasic applications for years and never ever used the debugging in MapBasic. I have basically learned to live with my own debug methods thru Print and Note statements – Peter Horsbøll Møller Feb 2 '14 at 18:49

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