Merge pull request #213 from certik/cond

Initial draft of conditional expression intrinsic paper

Author: certik

proposals/conditional_expressions/21-165.txt

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+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
+
+