To: J3                                                     J3/19-187r1
From:    Malcolm Cohen
Subject: Rejected Request for interpretation: REDUCE in constant expressio
Date: 2019-August-05


0. Reason for rejection

A reference to the REDUCE intrinsic function is not permitted in a
constant expression, because it cannot meet the requirements for
constant expressions.

That is, rule (6) of constant expressions says
  "a reference to a standard intrinsic function that is transformational
   ..., where each argument is a constant expression".

Although the REDUCE intrinsic function is transformational, the OPERATION
argument must be a procedure designator, in particular, a function
designator. A function designator is not a constant expression as it does
not satisfy any of the rules.

The original text of 19-187 appears below, unchanged.

===========================================================================

1. Introduction
The OPERATION argument of the REDUCE intrinsic function is permitted
to be a user defined function that meets the specified requirements.
A constant expression is an expression that can be used as an
initializer, in declaring a named constant, or as a kind type
parameter. Constant expressions in previous revisions of the
standard could be evaluated at compile time.

There is no restriction preventing the use of the REDUCE intrinsic
function in a constant expression. The lack of this restriction
permit expressions that result in invocation of user functions in
constant expressions.

2. Interpretation Request

----------------------------------------------------------------------

NUMBER: F18/xxx
TITLE: REDUCE intrinsic in constant expressions
KEYWORDS: REDUCE, constant expression
DEFECT TYPE: Erratum
STATUS: Submitted

Section 10.1.2 Constant expression states:

 "A constant expression is an expression with limitations that make
  it suitable for use as a kind type parameter, initializer, or
  named constant.  It is an expression in which each operation is
  intrinsic, and each primary is
  ...
  (6) a reference to a standard intrinsic function that is trans-
      formatoinal other than COMMAND_ARGUMENT_COUNT, GET_TEAM, NULL,
      NUM_IMAGES, TEAM_NUMBER, THIS_IMAGE, or TRANSFER, where each
      argument is a constant expression."

Question:

REDUCE is a transformation function that can invoke a user function.
Was it intended that REDUCE be allowed in a constant expression?

Answer:

No, it was an oversight to allow REDUCE in a constant expression.
An edit is provided to fix this oversight.

Edit:

[158:32]
  after "NUM_IMAGES,"
  insert " REDUCE,"
  to make the entire list item read
  "(6) a reference to a standard intrinsic function that is trans-
       formational, other than COMMAND_ARGUMENT_COUNT, GET_TEAM,
       NULL, NUM_IMAGES, REDUCE, TEAM_NUMBER, THIS_IMAGE, or
       TRANSFER, where each argument is a constant expression."

SUBMITTED BY: Jon Steidel

HISTORY: 19-xxx   m219  Submitted

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