Format specifications are strings with "%" markers indicating format placeholders. Format placeholders consist of:

    %[flags][width][.precision][type]
 

where the type is interpreted as follows:

    %b:         bool, formatted as "true" or "false"
    %s:         string, formatted as its unescaped contents
    %d, %i:     any basic integer type formatted as a decimal integer, signed if the basic integer type is signed.
    %u:         any basic integer type formatted as an unsigned decimal integer
    %x, %X, %o: any basic integer type formatted as an unsigned hexadecimal 
                (a-f)/Hexadecimal (A-F)/Octal integer
 
    %e, %E, %f, %F, %g, %G: 
                any basic floating point type (float,float32) formatted
                using a C-style floating point format specifications, i.e
 
          %e, %E: Signed value having the form [-]d.dddde[sign]ddd where 
                  d is a single decimal digit, dddd is one or more decimal
                  digits, ddd is exactly three decimal digits, and sign 
                  is + or -
 
          %f:     Signed value having the form [-]dddd.dddd, where dddd is one
                  or more decimal digits. The number of digits before the 
                  decimal point depends on the magnitude of the number, and 
                  the number of digits after the decimal point depends on 
                  the requested precision.
 
          %g, %G: Signed value printed in f or e format, whichever is 
                  more compact for the given value and precision.
 
 
    %M:     System.Decimal value
 
    %O:     Any value, printed by boxing the object and using it's ToString method(s)
 
    %A:     Any value, printed by using Microsoft.FSharp.Text.StructuredFormat.Display.any_to_string with the default layout settings 
 
    %a: a general format specifier, requires two arguments:
          (1) a function which accepts two arguments:
                (a) a context parameter of the appropriate type for the
                    given formatting function (e.g. an #System.IO.TextWriter)
                (b) a value to print
               and which either outputs or returns appropriate text.
 
          (2) the particular value to print
 
 
    %t: a general format specifier, requires one argument:
          (1) a function which accepts a context parameter of the
              appropriate type for the given formatting function (e.g. 
              an #System.IO.TextWriter)and which either outputs or returns 
              appropriate text.

 Basic integer types are:
    byte,sbyte,int16,uint16,int32,uint32,int64,uint64,nativeint,unativeint
 Basic floating point types are:
    float, float32

 
 The following format patterns are accepted but a warning is printed:

    %h(d|u|x|X|o)
    %l(d|u|x|X|o)

 The following format patterns are now deprecated:

    %Ld, %Li, %Lu, %Lx, %LX, %Lo: same, but an int64
    %nd, %ni, %nu, %nx, %nX, %no: same, but a nativeint
    %Ud, %Ui, %Uu, %Ux, %UX, %Uo: same, but an unsigned int32 (uint32)
    %ULd, %ULi, %ULu, %ULx, %ULX, %ULo: same, but an unsigned int64 (uint64)
    %Und, %Uni, %Unu, %Unx, %UnX, %Uno: same, but an unsigned nativeint 
                                        (unativeint)
 

Valid flags are:

     0: add zeros instead of spaces to make up the required width
     '-': left justify the result within the width specified
     '+': add a '+' character if the number is positive (to match a '-' sign 
          for negatives)
     ' ': add an extra space if the number is positive (to match a '-' 
              sign for negatives)
 

The printf '#' flag is not supported in this release and a compile-time error will be reported if it is used.