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A.8.3 Sequential Input-Output Operations

Static Semantics

The operations available for sequential input and output are described in this subclause. The exception Status_Error is propagated if any of these operations is attempted for a file that is not open. 
procedure Read(File : in File_Type; Item : out Element_Type);
Operates on a file of mode In_File. Reads an element from the given file, and returns the value of this element in the Item parameter. 
Discussion: We considered basing Sequential_IO.Read on Element_Type'Read from an implicit stream associated with the sequential file. However, Element_Type'Read is a type-related attribute, whereas Sequential_IO should take advantage of the particular constraints of the actual subtype corresponding to Element_Type to minimize the size of the external file. Furthermore, forcing the implementation of Sequential_IO to be based on Element_Type'Read would create an upward incompatibility since existing data files written by an Ada 83 program using Sequential_IO might not be readable by the identical program built with an Ada 95 implementation of Sequential_IO.
An Ada 95 implementation might still use an implementation-defined attribute analogous to 'Read to implement the procedure Read, but that attribute will likely have to be subtype-specific rather than type-related, and it need not be user-specifiable. Such an attribute will presumably be needed to implement the generic package Storage_IO (see A.9). 
The exception Mode_Error is propagated if the mode is not In_File. The exception End_Error is propagated if no more elements can be read from the given file. The exception Data_Error can be propagated if the element read cannot be interpreted as a value of the subtype Element_Type (see A.13, “Exceptions in Input-Output”).
Discussion: Data_Error need not be propagated if the check is too complex. See A.13, “Exceptions in Input-Output”. 
procedure Write(File : in File_Type; Item : in Element_Type);
Operates on a file of mode Out_File or Append_File. Writes the value of Item to the given file.
The exception Mode_Error is propagated if the mode is not Out_File or Append_File. The exception Use_Error is propagated if the capacity of the external file is exceeded.
function End_Of_File(File : in File_Type) return Boolean;
{AI05-0264-1} Operates on a file of mode In_File. Returns True if no more elements can be read from the given file; otherwise, returns False.
The exception Mode_Error is propagated if the mode is not In_File. 

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