J3/10-182r1
To: J3
From: Malcolm Cohen
Subject: Final interp 6: PURE FUNCTION results
Date: 2010 June 16

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NUMBER: F08/0032
TITLE: PURE FUNCTION result finalization
KEYWORDS: PURE FUNCTION FINAL
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress

QUESTION:

Consider
  MODULE m
    TYPE t
      REAL c
    CONTAINS
      FINAL f
    END TYPE
    INTEGER :: fcount = 0
  CONTAINS
    SUBROUTINE f(x)
      TYPE(t),INTENT(INOUT) :: x
      x%c = 0
      fcount = fcount + 1
    END SUBROUTINE
    PURE TYPE(t) FUNCTION f(a)
      REAL,INTENT(IN) :: a
      f%c = a
    END FUNCTION
  END MODULE

Even though this function is PURE, invoking it will inevitably result
in the execution of the impure FINAL subroutine f.  Thus, it cannot be
used within another PURE procedure or from within a DO CONCURRENT
loop, though it can be used from within a FORALL (because the
finalizations only get done on termination of the outermost FORALL).

Some other cases of impure finalization are prohibited by C1284, but
not this because it does not occur "in" the procedure.

Should an impurely-finalizable function result be allowed for a pure
function?

ANSWER:

No, this should not be allowed.  An edit is supplied to correct this
oversight in the requirements for pure procedures.

EDITS to 10-007:

[312:20+] In 12.7 after C1276, insert new constraint
  "C1276a The result variable of a pure function shall not be such
          that finalization of a reference to the function would
          reference an impure procedure."

SUBMITTED BY: Malcolm Cohen

HISTORY: 10-182   m192 F08/0032 Submitted
         10-182r1 m192 Revised.

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