Conditional Expressions

[certik]

Relevant papers:

• https://j3-fortran.org/doc/year/18/18-274.txt
• https://j3-fortran.org/doc/year/19/19-258.txt
• https://j3-fortran.org/doc/year/19/19-247r1.txt
• https://j3-fortran.org/doc/year/19/19-248.txt

Taking the second example from the 18-274 paper:

  IF (PRESENT(D)) THEN
    CALL SUB(A,B,C,D)
  ELSE IF (X<1) THEN
    CALL SUB(A,B,C,EPSILON(X))
  ELSE
    CALL SUB(A,B,C,SPACING(X))
  END IF

One proposed syntax is "keyword syntax":

CALL SUB(A, B, C, IF (PRESENT(D) THEN D ELSE IF (X < 1) THEN EPSILON(X) ELSE SPACING(X) END IF)

The second proposed syntax is "? syntax":

CALL SUB(A, B, C, ? (PRESENT(D) D :? (X < 1) EPSILON(X) : SPACING(X) ?)

[certik]

Which of these do people find the most readable and least readable?

My own feeling (from most readable to least readable):

1. Original
2. keyword syntax
3. ? syntax

We should also consider more use cases. I also asked at https://fortran-lang.discourse.group/t/202x-feature-conditional-expressions/329 to get more feedback on this feature.

[dev-zero]

To be honest I find both variants suboptimal (the first one would make an if/else/endif block return a value when being used inline, and simply run code as usual when not, but would be easier to read) and I wonder whether there is really a need for general conditional expressions or whether a ternary operator would be sufficient. In the latter case I would suggest the Python-based syntax (which one could even nest):

CALL SUB(A, B, C, D IF PRESENT(D) ELSE (EPSILON(X) IF (X < 1) ELSE SPACING(X)))

[certik]

@dev-zero in your opinion, can you rate by readability / preference the three options above plus your proposed Python-based syntax?

The ternary operator (that you presumably like) is the "? syntax" which is one of the proposed ideas, but you also say they are "suboptimal", so I am confused.

[milancurcic]

From most to least readable:

1. Original
2. Python/@dev-zero syntax
3. Keyword syntax
4. ? syntax

Caveat: I use 2 in Python a lot so I'm used to it, thus my preference.

[epagone]

FWIW my preference from most to least readable is:

1. Original
2. keyword syntax
3. ? syntax

Aside the reduced readability, IMHO I cannot see any practical advantage in the new proposed expressions.

[dev-zero]

My preference in terms of readability:

1. Original
2. Python-inspired syntax
3. Keyword syntax
4. ? syntax

As for why I dislike the ? syntax and the difference to the Python-inspired operator:

A general start and end marker ? for conditional expression would to me indicate that more complex conditional expressions should be possible at some point, with the only condition that all codepaths inside it have to return a compatible type, but that is with the current proposal not the case (nor should it be). On the other hand does the nesting as shown in the example above actually create (at least visually) an alias for ELSE IF: the :?, since the expression should actually be:

CALL SUB(A, B, C, ? (PRESENT(D)) D : ? (X < 1) EPSILON(X) : SPACING(X) ? ?)

correct? With the second to last ? omitted? Which I would find even less readable.

[everythingfunctional]

I think the readability question might slightly be missing the point. The question isn't about how easy is it to see what the code is doing, but how easy is it to see the code's intent.

To me, an if-else block says the intent of the code is to do different things based on some conditions. The new feature allows one to specify the intent "this variable (or argument) depends on some condition(s)" more clearly.

To use the example from above, if I see

  IF (PRESENT(D)) THEN
    CALL SUB(A,B,C,D)
  ELSE IF (X<1) THEN
    CALL SUB(A,B,C,EPSILON(X))
  ELSE
    CALL SUB(A,B,C,SPACING(X))
  END IF

It's not immediately obvious (especially in codes that are more complicated than this example) that we are definitely going to be calling sub in all cases, and the only difference is the value of the last argument. Whereas with

call SUB(A, B, C, IF (PRESENT(D)) THEN (D) ELSE IF (X < 1) THEN (EPSILON(X)) ELSE (SPACING(X) END IF)

that intent is put front and center without having to compare each branch in an entire if-else block.

So, my preferences would be for

1. Keyword syntax - most similar to existing Fortran syntax
2. Python syntax - still quite likely to be easily understood by new programmers
3. ? syntax - similar to other languages, but not immediately obvious to new programmers

[certik]

If the idea is to call SUB just once, to make the intent obvious, as @everythingfunctional correctly points out, then here is the alternative to the original:

  IF (PRESENT(D)) THEN
    D2 = D
  ELSE IF (X<1) THEN
    D2 = EPSILON(X)
  ELSE
    D2 = SPACING(X)
  END IF
  CALL SUB(A,B,C,D2)

Which I personally find more readable than:

CALL SUB(A, B, C, IF (PRESENT(D) THEN D ELSE IF (X < 1) THEN EPSILON(X) ELSE SPACING(X) END IF)

But it is true you have to declare an extra variable (although the performance of the code should be identical with a good compiler).

[milancurcic]

Is a subset of this a good candidate for a stdlib function if_then_else(condition, expression1, expression2)?

[everythingfunctional]

Is a subset of this a good candidate for a stdlib function if_then_else(condition, expression1, expression2)?

It would be, but merge already would be sufficient, and the idea of the proposal is to not evaluate the unused expression (which merge can't do).

[everythingfunctional]

@certik , while you've removed the difficulty of seeing "do we always call SUB in every case", you've now introduced the ambiguity of "do we always assign to D2 in every case?" You're back to the exact same problem. You're using a multi-statement block to perform a single logical operation. A reader must inspect and evaluate every branch (the block as a whole) to understand it is performing a single logical operation.

[certik]

My understanding is that this feature was approved by WG5 for inclusion into 202X based on a survey, where people expressed a wish to have conditional expressions in Fortran, but I don't think there were concrete proposals how it would look like, just that the feature would be nice to have. @sblionel is that an accurate statement?

@everythingfunctional explained well the main argument for conditional expressions is that they enforce a single logical operation to be assigned somewhere (as the result of the conditional expression).

I agree that at this level of "requirements" it seems like a good idea and I am not against that (I use it sometimes in Python, although rarely; I never use the ? notation in C or C++). But when it comes down to syntax and how it would actually look like in Fortran, it seems less readable in practice to many people, and that is would should count. If a feature looks good in abstract terms, but does not look good in concrete (syntax) terms, in my opinion we should not put it in until it looks good both in abstract and concrete terms.

A prior compiler implementation of this would be very helpful, so that we can play with it more, before putting it into the language.

[pbrady]

Adopting ?: for a conditional expression but using a difference syntax from those languages which already use it for the same purpose (i.e. C, C++, Java, Javascript), is a bad idea and will only lead to confusion. As someone who uses multiple languages, I would appreciate it if Fortran did not do anything weird here and go off the beaten path for no apparent reason.