To: J3                                                     J3/21-165
From: Ondrej Certik, Milan Curcic, Zach Jibben
Subject: Conditional expressions using intrinsic function
Date: 2021-June-27
Reference: 21-157, 21-159

Paper 21-157 proposes two syntactic forms for conditional expressions. The
paper 21-159 proposes an alternative form. This paper proposes to implement
conditional expressions using an intrinsic function.

The syntactic forms in 21-157 are too verbose, as argued in 21-159. The
form in 21-159 is concise, but based on the poll [1], it is not the most
popular either.

We propose the following syntactic form

   <cond-expr> is ifthen( predicate, consequent, alternative )

where the `predicate` is a scalar logical expression and the `consequent`
and `alternative` are compatible expressions. The `alternative` can be
another `ifthen` as shown in an example below.

Examples:

      res = ifthen(x >= 0.0, sqrt(x), -0.0)

      res = ifthen(present(x), a, ifthen(present(b), b, 0))

Objections raised in the past:

1) It is harder to do chaining due to the extra parenthesis at the end.

Answer: it seems in practice it is not too bad, as in the second example
above.  Here is a longer chain:

      res = ifthen(present(x), x, ifthen(present(y), y,
                     ifthen(present(z), z, 0)))

2) `ifthen` does not behave like a regular function, it is effectively new
syntax, so we might as well actually introduce new syntax, to make it less
confusing

There is a precedent in optional arguments, where the argument is not
evaluated but passed through through nested functions until it hits the
function `present`. In a similar way, the arguments to `ifthen` are not
evaluated ahead of time, but rather passed into the function `ifthen`
itself, similar to how `present(x)` works. Both `ifthen` and `present` must
be implemented by the compiler itself, they cannot be implemented in
standard Fortran code.

References:

[1] https://fortran-lang.discourse.group/t/
    poll-fortran-202x-conditional-expressions-syntax/1425