Fortran 2008 Overview

Fortran 2008 Overview

Table of Contents

1 Introduction

This document describes those parts of the Fortran 2008 language which are not in Fortran 2003, and which are supported by the latest release of the NAG Fortran Compiler.

The compiler release in which a feature was made available is indicated by square brackets; for example, a feature marked as ‘[5.3]’ was first available in Release 5.3.

2 Overview of Fortran 2008

The new features of Fortran 2008 that are supported by the NAG Fortran Compiler can be grouped as follows:

  • data declaration;
  • data usage and computation;
  • execution control;
  • intrinsic procedures and modules;
  • input/output extensions;
  • programs and procedures.

3 Data declaration [mostly 6.0]

  • The maximum rank of an array has been increased from 7 to 15. For example,
       REAL array(2,2,2,2,2,2,2,2,2,2,2,2,2,2,2)
    declares a 15-dimensional array.
  • [3.0] 64-bit integer support is required, that is, the result of SELECTED_INT_KIND(18) is a valid integer kind number.
  • A named constant (PARAMETER) that is an array can assume its shape from its defining expression; this is called an implied-shape array. The syntax is that the upper bound of every dimension must be an asterisk, for example
       REAL,PARAMETER :: idmat3(*,*) = Reshape( [ 1,0,0,0,1,0,0,0,1 ], [ 3,3 ] )
       REAL,PARAMETER :: yeardata(2000:*) = [ 1,2,3,4,5,6,7,8,9 ]
    declares idmat3 to have the bounds (1:3,1:3), and yeardata to have the bounds (2000:2008).
  • The TYPE keyword can be used to declare entities of intrinsic type, simply by putting the intrinsic type-spec within the parentheses. For example,
       TYPE(REAL) x
       TYPE(COMPLEX(KIND(0d0))) y
       TYPE(CHARACTER(LEN=80)) z
    is completely equivalent, apart from being more confusing, to
       REAL x
       COMPLEX(KIND(0d0)) y
       CHARACTER(LEN=80) z
  • As a consequence of the preceding extension, it is no longer permitted to define a derived type that has the name DOUBLEPRECISION.
  • [5.3] A type-bound procedure declaration statement may now declare multiple type-bound procedures. For example, instead of
          PROCEDURE,NOPASS :: a
          PROCEDURE,NOPASS :: b=>x
          PROCEDURE,NOPASS :: c
    the single statement
          PROCEDURE,NOPASS :: a, b=>x, c
    will suffice.

4 Data usage and computation [mostly 5.3]

  • In a structure constructor, the value for an allocatable component may be omitted: this has the same effect as specifying NULL().
  • [6.0] When allocating an array with the ALLOCATE statement, if SOURCE= or MOLD= is present and its expression is an array, the array can take its shape directly from the expression. This is a lot more concise than using SIZE or UBOUND, especially for a multi-dimensional array.

    For example,

       SUBROUTINE s(x,mask)
          REAL x(:,:,:)
          LOGICAL mask(:,:,:)
          REAL,ALLOCATABLE :: y(:,:,:)
          WHERE (mask)
            y = 1/x
            y = HUGE(x)
          END WHERE
          ! ...
  • [6.1] The real and imaginary parts of a COMPLEX object can be accessed using the complex part designators ‘%RE’ and ‘%IM’. For example, given
       COMPLEX,PARAMETER :: c = (1,2), ca(2) = [ (3,4),(5,6) ]
    the designators c%re and c%im have the values 1 and 2 respectively, and ca%re and ca%im are arrays with the values [ 3,5 ] and [ 4,6 ] respectively. In the case of variables, for example
       COMPLEX :: v, va(10)
    the real and imaginary parts can also be assigned to directly; the statement
       va%im = 0
    will set the imaginary part of each element of va to zero without affecting the real part.
  • In an ALLOCATE statement for one or more variables, the MOLD= clause can be used to give the variable(s) the dynamic type and type parameters (and optionally shape) of an expression. The expression in MOLD= must be type-compatible with each allocate-object, and if the expression is a variable (e.g. MOLD=X), the variable need not be defined. Note that the MOLD= clause may appear even if the type, type parameters and shape of the variable(s) being allocated are not mutable. For example,
       CLASS(*),POINTER :: a,b,c
    will allocate the unlimited polymorphic pointers A, B and C to be of type Integer (with default kind); unlike SOURCE=, the values of A, B and C will be undefined.
  • [5.3.1] Assignment to a polymorphic allocatable variable is permitted. If the variable has different dynamic type or type parameters, or if an array, a different shape, it is first deallocated. If it is unallocated (or is deallocated by step 1), it is then allocated to have the correct type and shape. It is then assigned the value of the expression. Note that the operaton of this feature is similar to the way that ALLOCATE(variable,SOURCE=expr) works. For example, given
       CLASS(*),ALLOCATABLE :: x
    execution of the assignment statement
       x = 43
    will result in X having dynamic type Integer (with default kind) and value 43, regardless of whether X was previously unallocated or allocated with any other type (or kind).
  • [6.1] Rank-remapping pointer assignment is now permitted when the target has rank greater than one, provided it is “simply contiguous” (a term which means that it must be easily seen at compile-time to be contiguous). For example, the pointer assignment in
       REAL,TARGET :: x(100,100)
       REAL,POINTER :: x1(:)
       x1(1:Size(x)) => x
    establishes X1 as a single-dimensional alias for the whole of X.

5 Execution control [mostly 6.0]

  • [5.3] The BLOCK construct allows declarations of entities within executable code. For example,
      Do i=1,n
          Real tmp
          tmp = a(i)**3
          If (tmp>b(i)) b(i) = tmp
        End Block
      End Do
    Here the variable tmp has its scope limited to the BLOCK construct, so will not affect anything outside it. This is particularly useful when including code by INCLUDE or by macro preprocessing.

    All declarations are allowed within a BLOCK construct except for COMMON, EQUIVALENCE, IMPLICIT, INTENT, NAMELIST, OPTIONAL and VALUE; also, statement function definitions are not permitted.

    BLOCK constructs may be nested; like other constructs, branches into a BLOCK construct from outside are not permitted. A branch out of a BLOCK construct “completes” execution of the construct.

    Entities within a BLOCK construct that do not have the SAVE attribute (including implicitly via initialisation), will cease to exist when execution of the construct is completed. For example, an allocated ALLOCATABLE variable will be automatically deallocated, and a variable with a FINAL procedure will be finalised.

  • The EXIT statement is no longer restricted to exiting from a DO construct; it can now be used to jump to the end of a named ASSOCIATE, BLOCK, IF, SELECT CASE or SELECT TYPE construct (i.e. any named construct except FORALL and WHERE). Note that an EXIT statement with no construct-name still exits from the innermost DO construct, disregarding any other named constructs it might be within.
  • In a STOP statement, the stop-code may be any scalar constant expression of type integer or default character. (In the NAG Fortran Compiler this also applies to the PAUSE statement, but that statement is no longer standard Fortran.) Additionally, the STOP statement with an integer stop-code now returns that value as the process exit status (on most operating systems there are limits on the value that can be returned, so for the NAG Fortran Compiler this returns only the lower eight bits of the value).
  • The ERROR STOP statement has been added. This is similar to the STOP statement, but causes error termination rather than normal termination. The syntax is identical to that of the STOP statement apart from the extra keyword ‘ERROR’ at the beginning. Also, the default process exit status is zero for normal termination, and non-zero for error termination.

    For example,

       IF (x<=0) ERROR STOP 'x must be positive'
  • [6.1] The FORALL construct now has an optional type specifier in the initial statement of the construct, which can be used to specify the type (which must be INTEGER) and kind of the index variables. When this is specified, the existence or otherwise of any entity in the outer scope that has the same name as an index variable does not affect the index variable in any way. For example,
      Complex i(100)
      Real x(200)
      Forall (Integer :: i=1:Size(x)) x(i) = i

    Note that the FORALL construct is still not recommended for high performance, as the semantics imply evaluating the right-hand sides into array temps the size of the iteration space, and then assigning to the variables; this usually performs worse than ordinary DO loops.

  • [6.1] The DO CONCURRENT construct is a DO loop with restrictions and semantics intended to allow efficient execution. The iterations of a DO CONCURRENT construct may be executed in any order, and possibly even in parallel. The loop index variables are local to the construct.

    The DO CONCURRENT header has similar syntax to the FORALL header, including the ability to explicitly specify the type and kind of the loop index variables, and including the scalar mask.

    The restrictions on the DO CONCURRENT construct are:

    • no branch is allowed from within the construct to outside of it (this includes the RETURN and STOP statements, but ERROR STOP is allowed);
    • the EXIT statement cannot be used to terminate the loop;
    • the CYCLE statement cannot refer to an outer loop;
    • there must be no dependencies between loop iterations, and if a variable is assigned to by any iteration, it is not allowed to be referenced by another iteration unless that iteration assigns it a value first;
    • all procedures referenced within the construct must be pure;
    • no reference to IEEE_GET_FLAG or IEEE_SET_HALTING_MODE is allowed.

    For example,

      Integer vsub(n)
      Do Concurrent (i=1:n)
        ! Safe because vsub has no duplicate values.
        x(vsub(i)) = i
      End Do

    The full syntax of the DO CONCURRENT statement is:

    [ do-construct-name : ] DO [ label ] [ , ] CONCURRENT forall-header

    where forall-header is

    ( [ integer-type-spec :: ] triplet-spec [ , triplet-spec ]... [ , mask-expr ] )

    where mask-expr is a scalar logical expression, and triplet-spec is

    name = expr : expr [ : expr ]

6 Intrinsic procedures and modules

6.1 Additional mathematical intrinsic functions [mostly 5.3.1]

  • The elemental intrinsic functions ACOSH, ASINH and ATANH compute the inverse hyperbolic cosine, sine or tangent respectively. There is a single argument X, which may be of type Real or Complex; the result of the function has the same type and kind. When the argument is Complex, the imaginary part is expressed in radians and lies in the range 0≤im≤π for the ACOSH function, and −π/2≤im≤π/2 for the ASINH and ATANH functions.

    For example, ACOSH(1.543081), ASINH(1.175201) and ATANH(0.7615942) are all approximately equal to 1.0.

  • [6.1] The new elemental intrinsic functions BESSEL_J0, BESSEL_Y0, BESSEL_J1 and BESSEL_Y1 compute the Bessel functions J0, Y0, J1 and Y1 respectively. These functions are solutions to Bessel's differential equation. The J functions are of the 1st kind and the Y functions are of the 2nd kind; the following subscript indicates the order (0 or 1). There is a single argument X, which must be of type Real; the result of the function has the same type and kind. For functions of the 2nd kind (BESSEL_Y0 and BESSEL_Y1), the argument X must be positive. For example, BESSEL_J0(1.5) is approximately 0.5118276, BESSEL_Y0(1.5) is approximately 0.3824489, BESSEL_J1(1.5) is approximately 0.5579365 and BESSEL_Y1(1.5) is approximately -0.4123086.
  • [6.1] The new intrinsic functions BESSEL_JN and BESSEL_YN compute the Bessel functions Jn and Yn respectively. These functions come in two forms: an elemental form and a transformational form.

    The elemental form has two arguments: N, the order of the function to compute, and X, the argument of the Bessel function. BESSEL_JN(0,X) is identical to BESSEL_J0(X), etc..

    The transformational form has three scalar arguments: N1, N2 and X. The result is a vector of size MAX(N2-N1+1,0), containing approximations to the Bessel functions of orders N1 to N2 applied to X.

    For example, BESSEL_JN(5,7.5) is approximately 0.283474, BESSEL_YN(5,7.5) is approximately 0.175418, BESSEL_JN(3,5,7.5) is approximately [ -0.258061, 0.023825, 0.283474 ] and BESSEL_YN(3,5,7.5) is approximately [ 0.159708, 0.314180, 0.175418 ].

  • [6.0] The elemental intrinsic functions ERF, ERFC and ERFC compute the error function, the complementary error function and the scaled complementary error function, respectively. The single argument X must be of type real.

    The error function is the integral of −t2 from 0 to X, times 2/SQRT(π); this rapidly converges to 1. The complementary error function is 1 minus the error function, and fairly quickly converges to zero. The scaled complementary error function scales the value (of 1 minus the error function) by EXP(X**2); this also converges to zero but only very slowly.

  • [6.0] The elemental intrinsic functions GAMMA and LOG_GAMMA compute the gamma function and the natural logarithm of the absolute value of the gamma function respectively. The single argument X must be of type real, and must not be zero or a negative integer.

    The gamma function is the extension of factorial from the integers to the reals; for positive integers, GAMMA(X) is equal to (X−1)!, i.e. factorial of X−1. This grows very rapidly and thus overflows for quite small X; LOG_GAMMA also diverges but much more slowly.

  • The elemental intrinsic function HYPOT computes the “Euclidean distance function” (square root of the sum of squares) of its arguments X and Y without overflow or underflow for very large or small X or Y (unless the result itself overflows or underflows). The arguments must be of type Real with the same kind, and the result is of type Real with that kind. Note that HYPOT(X,Y) is semantically and numerically equal to ABS(CMPLX(X,Y,KIND(X))).

    For example, HYPOT(3e30,4e30) is approximately equal to 5e30.

  • The array reduction intrinsic function NORM2(X,DIM) reduces Real arrays using the L2-norm operation. This operates exactly the same as SUM and PRODUCT, except for the operation involved. The L2 norm of an array is the square root of the sum of the squares of the elements. Note that unlike most of the other reduction functions, NORM2 does not have a MASK argument. The DIM argument is optional; an actual argument for DIM is not itself permitted to be an optional dummy argument.

    The calculation of the result value is done in such a way as to avoid intermediate overflow and underflow, except when the result itself is outside the maximum range. For example, NORM2([X,Y]) is approximately the same as HYPOT(X,Y).

6.2 Additional intrinsic functions for bit manipulation [mostly 5.3]

  • The elemental intrinsic functions BGE, BGT, BLE and BLT perform bitwise (i.e. unsigned) comparisons. They each have two arguments, I and J, which must be of type Integer but may be of different kind. The result is default Logical.

    For example, BGE(INT(Z'FF',INT8),128) is true, while INT(Z'FF',INT8)>=128 is false.

  • [5.3.1] The elemental intrinsic functions DSHIFTL and DSHIFTR perform double-width shifting. They each have three arguments, I, J and SHIFT which must be of type Integer, except that one of I or J may be a BOZ literal constant – it will be converted to the type and kind of the other I or J argument. I and J must have the same kind if they are both of type Integer. The result is of type Integer, with the same kind as I and J. The I and J arguments are effectively concatenated to form a single double-width value, which is shifted left or right by SHIFT positions; for DSHIFTL the result is the top half of the combined shift, and for DSHIFTR the result is the bottom half of the combined shift.

    For example, DSHIFTL(INT(B'11000101',1),B'11001001',2) has the value INT(B'00010111',1) (decimal value 23), whereas DSHIFTR(INT(B'11000101',1),B'11001001',2) has the value INT(B'01110010',1) (decimal value 114).

  • The array reduction intrinsic functions IALL, IANY and IPARITY reduce arrays using bitwise operations. These are exactly the same as SUM and PRODUCT, except that instead of reducing the array by the + or * operation, they reduce it by the IAND, IOR and IEOR intrinsic functions respectively. That it, each element of the result is the bitwise-and, bitwise-or, or bitwise-exclusive-or of the reduced elements. If the number of reduced elements is zero, the result is zero for IANY and IPARITY, and NOT(zero) for IALL.
  • The elemental intrinsic functions LEADZ and TRAILZ return the number of leading (most significant) and trailing (least significant) zero bits in the argument I, which must be of type Integer (of any kind). The result is default Integer.
  • The elemental intrinsic functions MASKL and MASKR generate simple left-justified and right-justified bitmasks. The value of MASKL(I,KIND) is an integer with the specified kind that has its leftmost I bits set to one and the rest set to zero; I must be non-negative and less than or equal to the bitsize of the result. If KIND is omitted, the result is default integer. The value of MASKR is similar, but has its rightmost I bits set to one instead.
  • [5.3.1] The elemental intrinsic function MERGE_BITS(I,J,MASK) merges the bits from Integer values I and J, taking the bit from I when the corresponding bit in MASK is 1, and taking the bit from J when it is zero. All arguments must be BOZ literal constants or of type Integer, and all the Integer arguments must have the same kind; at least one of I and J must be of type Integer, and the result has the same type and kind.

    Note that MERGE_BITS(I,J,MASK) is identical to IOR(IAND(I,MASK),IAND(J,NOT(MASK))).

    For example, MERGE_BITS(INT(B'00110011',1),B'11110000',B'10101010') is equal to INT(B'01110010') (decimal value 114).

  • The array reduction intrinsic function PARITY reduces Logical arrays. It is exactly the same as ALL and ANY, except that instead of reducing the array by the .AND. or .OR. operation, it reduces it by the .NEQV. operation. That is, each element of the result is .TRUE. if an odd number of reduced elements is .TRUE..
  • The elemental intrinsic function POPCNT(I) returns the number of bits in the Integer argument I that are set to 1. The elemental intrinsic function POPPAR(I) returns zero if the number of bits in I that are set to 1 are even, and one if it is odd. The result is default Integer.

6.3 Other new intrinsic procedures [mostly 5.3.1]

  • The intrinsic subroutine EXECUTE_COMMAND_LINE passes a command line to the operating system's command processor for execution. It has five arguments, in order these are:
    CHARACTER(*),INTENT(IN) :: COMMAND — the command to be executed;
    LOGICAL,INTENT(IN),OPTIONAL :: WAIT — whether to wait for command completion (default true);
    INTEGER,INTENT(INOUT),OPTIONAL :: EXITSTAT — the result value of the command;
    CHARACTER(*),INTENT(INOUT),OPTIONAL :: CMDMSG — the error message if CMDSTAT is non-zero.

    CMDSTAT values are zero for success, −1 if command line execution is not supported, −2 if WAIT is present and false but asynchronous execution is not supported, and a positive value to indicate some other error. If CMDSTAT is not present but would have been set non-zero, the program will be terminated. Note that Release 5.3.1 supports command line execution on all systems, and does not support asynchronous execution on any system.

    For example, CALL EXECUTE_COMMAND_LINE('echo Hello') will probably display ‘Hello’ in the console window.

  • The intrinsic function STORAGE_SIZE(A,KIND) returns the size in bits of a scalar object with the same dynamic type and type parameters as A, when it is stored as an array element (i.e. including any padding). The KIND argument is optional; the result is type Integer with kind KIND if it is present, and default kind otherwise.

    If A is allocatable or a pointer, it does not have to be allocated unless it has a deferred type parameter (e.g. CHARACTER(:)) or is CLASS(*). If it is a polymorphic pointer, it must not have an undefined status.

    For example, STORAGE_SIZE(13_1) is equal to 8 (bits).

  • [6.0] The intrinsic inquiry function IS_CONTIGUOUS has a single argument ARRAY, which can be an array of any type. The function returns true if ARRAY is stored contiguously, and false otherwise. Note that this question has no meaning for an array with no elements, or for an array expression since that is a value and not a variable.

6.4 Changes to existing intrinsic procedures [5.3.1]

  • The intrinsic functions ACOS, ASIN, ATAN, COSH, SINH, TAN and TANH now accept arguments of type Complex. Note that the hyperbolic and non-hyperbolic versions of these functions and the new ACOSH, ASINH and ATANH functions are all related by simple algebraic identities, for example the new COSH(X) is identical to the old COS((0,1)*X) and the new SINH(X) is identical to the old (0,-1)*SIN((0,1)*X).
  • The intrinsic function ATAN now has an extra form ATAN(Y,X), with exactly the same semantics as ATAN2(Y,X).
  • The intrinsic function SELECTED_REAL_KIND now has a third argument RADIX; this specifies the desired radix of the Real kind requested. Note that the function IEEE_SELECTED_REAL_KIND in the intrinsic module IEEE_ARITHMETIC also has this new third argument, and will allow requesting IEEE decimal floating-point kinds if they become available in the future.

6.5 ISO_FORTRAN_ENV additions

[5.3] The standard intrinsic module ISO_FORTRAN_ENV contains additional named constants as follows.
  • The additional scalar integer constants INT8, INT16, INT32, INT64, REAL32, REAL64 and REAL128 supply the kind type parameter values for integer and real kinds with the indicated bit sizes.
  • The additional named array constants CHARACTER_KINDS, INTEGER_KINDS, LOGICAL_KINDS and REAL_KINDS list the available kind type parameter values for each type (in no particular order).

[6.1] The standard intrinsic module ISO_FORTRAN_ENV contains two new functions as follows.

  • COMPILER_VERSION. This function is pure, has no arguments, and returns a scalar default character string that identifies the version of the compiler that was used to compile the source file. This function may be used in a constant expression, e.g. to initialise a variable or named constant with this information. For example,
      Module version_info
        Use Iso_Fortran_Env
        Character(Len(Compiler_Version())) :: compiler = Compiler_Version()
      End Module
      Program show_version_info
        Use version_info
        Print *,compiler
      End Program
    With release 6.1 of the NAG Fortran Compiler, this program will print something like
     NAG Fortran Compiler Release 6.1(Tozai) Build 6105
  • COMPILER_OPTIONS. This function is pure, has no arguments, and returns a scalar default character string that identifies the options supplied to the compiler when the source file was compiled. This function may be used in a constant expression, e.g. to initialise a variable or named constant with this information. For example,
      Module options_info
        Use Iso_Fortran_Env
        Character(Len(Compiler_Options())) :: compiler = Compiler_Options()
      End Module
      Program show_options_info
        Use options_info
        Print *,compiler
      End Program
    If compiled with the options -C=array -C=pointer -O, this program will print something like
     -C=array -C=pointer -O

7 Input/output extensions [mostly 5.3]

  • The NEWUNIT= specifier has been added to the OPEN statement; this allocates a new unit number that cannot clash with any other logical unit (the unit number will be a special negative value). For example,
          INTEGER unit
          WRITE(unit,*) 'Logfile opened.'
    The NEWUNIT= specifier can only be used if either the FILE= specifier is also used, or if the STATUS= specifier is used with the value 'SCRATCH'.
  • Recursive input/output is allowed on separate units. For example, in
       Write (*,Output_Unit) f(100)
    the function f is permitted to perform i/o on any unit except Output_Unit; for example, if the value 100 is out of range, it would be allowed to produce an error message with
       Write (*,Error_Unit) 'Error in F:',n,'is out of range'
  • [6.0] A sub-format can be repeated an indefinite number of times by using an asterisk (*) as its repeat count. For example,
       SUBROUTINE s(x)
         LOGICAL x(:)
         PRINT 1,x
    1    FORMAT('x =',*(:,' ',L1))
    will display the entire array x on a single line, no matter how many elements x has. An infinite repeat count is only allowed at the top level of the format specification, and must be the last format item.
  • [6.0] The G0 and G0.d edit descriptors perform generalised editing with all leading and trailing blanks (except those within a character value itself) omitted. For example,
       PRINT 1,1.25,.True.,"Hi !",123456789
    1  FORMAT(*(G0,','))
    produces the output
    1.250000,T,Hi !,123456789,

8 Programs and procedures [mostly 5.3]

  • An empty internal subprogram part, module subprogram part or type-bound procedure part is now permitted following a CONTAINS statement. In the case of the type-bound procedure part, an ineffectual PRIVATE statement may appear following the unnecessary CONTAINS statement.
  • [6.0] An internal procedure can be passed as an actual argument or assigned to a procedure pointer. When the internal procedure is invoked via the dummy argument or procedure pointer, it can access the local variables of its host procedure. In the case of procedure pointer assignment, the pointer is only valid until the host procedure returns (since the local variables cease to exist at that point).

    For example,

      SUBROUTINE mysub(coeffs)
        REAL,INTENT(IN) :: coeffs(0:) ! Coefficients of polynomial.
        REAL integral
        integral = integrate(myfunc,0.0,1.0) ! Integrate from 0.0 to 1.0.
        PRINT *,'Integral =',integral
        REAL FUNCTION myfunc(x) RESULT(y)
          REAL,INTENT(IN) :: x
          INTEGER i
          y = coeffs(UBOUND(coeffs,1))
          DO i=UBOUND(coeffs,1)-1,0,-1
            y = y*x + coeffs(i)
          END DO
  • The rules used for generic resolution and for checking that procedures in a generic are unambiguous have been extended. The extra rules are that
    • a dummy procedure is distinguishable from a dummy variable;
    • an ALLOCATABLE dummy variable is distinguishable from a POINTER dummy variable that does not have INTENT(IN).
  • [6.0] A disassociated pointer, or an unallocated allocatable variable, may be passed as an actual argument to an optional nonallocatable nonpointer dummy argument. This is treated as if the actual argument were not present.
  • [5.3.1] Impure elemental procedures can be defined using the IMPURE keyword. An impure elemental procedure has the restrictions that apply to elementality (e.g. all arguments must be scalar) but does not have any of the “pure” restrictions. This means that an impure elemental procedure may have side effects and can contain input/output and STOP statements. For example,
        Impure Elemental Integer Function checked_addition(a,b) Result(c)
          Integer,Intent(In) :: a,b
          If (a>0 .And. b>0) Then
            If (b>Huge(c)-a) Stop 'Positive Integer Overflow'
          Else If (a<0 .And. b<0) Then
            If ((a+Huge(c))+b<0) Stop 'Negative Integer Overflow'
          End If
          c = a + b
        End Function
    When an argument is an array, an impure elemental procedure is applied to each element in array element order (unlike a pure elemental procedure, which has no specified order). An impure elemental procedure cannot be referenced in a context that requires a procedure to be pure, e.g. within a FORALL construct.

    Impure elemental procedures are probably most useful for debugging (because i/o is allowed) and as final procedures.

  • [6.0] If an argument of a pure procedure has the VALUE attribute it does not need any INTENT attribute. For example,
       PURE SUBROUTINE s(a,b)
         REAL,INTENT(OUT) :: a
         REAL,VALUE :: b
         a = b

    Note however that the second argument of a defined assignment subroutine, and all arguments of a defined operator function, are still required to have the INTENT(IN) attribute even if they have the VALUE attribute.

  • [5.3.1] The FUNCTION or SUBROUTINE keyword on the END statement for an internal or module subprogram is now optional (when the subprogram name does not appear). Previously these keywords were only optional for external subprograms.
  • ENTRY statements are regarded as obsolescent.
  • [1.0] A line in the program is no longer prohibited from beginning with a semi-colon.

9 References

The Fortran 2008 standard, IS 1539-1:2010(E), is available from ISO as well as from many national standards bodies. A number of books describing the new standard are available; the recommended reference book is “Modern Fortran Explained” by Metcalf, Reid & Cohen, Oxford University Press, 2011 (ISBN 978-0-19-960141-7).