To: J3                                                     J3/21-159
From: Robert Corbett
Subject: Concise conditional expressions
References: 21-157
Date: 2021-June-23

Paper 21-157 proposes two syntactic forms for
conditional expressions.  This paper proposes
an alternative form.

I am not happy with either of the syntactic
forms for conditional expressions proposed in
paper 21-157.  The verbose form is far too
verbose.  The line noise form is better but
still too verbose, and it uses the character ?
which I prefer to leave unused.

I propose the following syntactic form

   <cond-expr> is ( predicate -> consequent
                    [ : predicate -> consequent ] ...
                      : alternative )

where the predicates are scalar logical expressions
and the consequents and alternative are compatible
expressions.

The semantics of this syntactic form map to the
semantics in the specs paper as follows

      ( predicate -> consequent
        [ : predicate -> consequent ] ...
          : alternative )

maps to

      if (predicate) then (consequent)
      [ elif (predicate) then (consequent) ] ...
      else (alternative) endif


Examples:

      ( x >= 0.0 -> sqrt(x) : -0.0 )

      ( present(a) -> a : present(b) -> b : 0 )