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Exception handling may be used to separate the detection of an error from the response to that error:
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package File_System is type File_Handle is limited private;
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File_Not_Found : exception; procedure Open(F : in out File_Handle; Name : String); −− raises File_Not_Found if named file does not exist
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End_Of_File : exception; procedure Read(F : in out File_Handle; Data : out Data_Type); −− raises End_Of_File if the file is not open
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... end File_System;
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package body File_System is procedure Open(F : in out File_Handle; Name : String) is begin if File_Exists(Name) then ... else raise File_Not_Found with "File not found: " & Name & "."; end if; end Open;
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procedure Read(F : in out File_Handle; Data : out Data_Type) is begin if F.Current_Position <= F.Last_Position then ... else raise End_Of_File; end if; end Read;
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...
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end File_System;
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with Ada.Text_IO; with Ada.Exceptions; with File_System; use File_System; use Ada; procedure Main is begin ... −− call operations in File_System exception when End_Of_File => Close(Some_File); when Not_Found_Error : File_Not_Found => Text_IO.Put_Line(Exceptions.Exception_Message(Not_Found_Error)); when The_Error : others => Text_IO.Put_Line("Unknown error:"); if Verbosity_Desired then Text_IO.Put_Line(Exceptions.Exception_Information(The_Error)); else Text_IO.Put_Line(Exceptions.Exception_Name(The_Error)); Text_IO.Put_Line(Exceptions.Exception_Message(The_Error)); end if; raise; end Main;
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In the above example, the File_System package contains information about detecting certain exceptional situations, but it does not specify how to handle those situations. Procedure Main specifies how to handle them; other clients of File_System might have different handlers, even though the exceptional situations arise from the same basic causes.