Re: Can .ads be compiled alone?

[rriehle@itu.edu]

On Thursday, November 6, 2014 9:21:38 PM UTC-8, moixa wrote:
> Newbie question.
> 
> I have only two files, machinery_1_3.ads [1] and machinery_1_3.adb [2], in some directory, when I `gnat compile machinery_1_3.ads`, it told me compile error, (but no further information).
> 
> A stackoverflow post[3] said that it's a good practice to separate compile the spec and the body, so, why I cannot compile my .ads?
> 
> ---------------------------
> 
> [3] http://stackoverflow.com/questions/9921794/error-when-compiling-spec-files
> 
> ---------------------------
> 
> [1] machinery_1_3.ads
> package Machinery_1_3 is                            -- 1 Package specification; requires body
>     type Machine is private;                        -- 2 Specifies the visible part of the data type;
>     procedure Turn_On (M : in out Machine);         -- 3 procedure specification
>     procedure Turn_Off (M : in out Machine);        -- 4 procedure specification
>     function Is_On (M : in Machine) return Boolean; -- 5 function specification
> private                                             -- 6 private part hidden from a client of contract
>     type Machine is record                          -- 7 full definition of the publicly declared type
>         Turned_On : Boolean := False;               -- 8 component of the type; OOP attribute
>     end record;                                     -- 9 scope terminator for the component
> end Machinery_1_3;                                  -- 10 scope terminator for the specification
> 
> ---------------------------
> 
> [2] machinery_1_3.adb
> package body Machinery_1_3 is                         -- 1 Package body; implements specification declarations
>     procedure Turn_On (M : in out Machine) is         -- 2 Repeat procedure specification; compiler checks this
>     begin                                             -- 3 Starts algorithmic section of procedure
>         M.Turned_ON := True;                          -- 4 Simple assignment statement of boolean value
>     end Turn_On;                                      -- 5 Procedure scope terminator is required
>     procedure Turn_Off (M : in out Machine) is        -- 6 Must match profile in specification
>     begin                                             -- 7 Algorithms between begin and end
>         M.Turned_On := False;                         -- 8 M.Turned called dot notation
>     end Turn_Off;                                     -- 9 Name is optional but end is required
>     function Is_On (M : in Machine) return Boolean is -- 10 In mode is like a constant; it may
>     begin                                             -- 11 not be on left side of assignment
>         return M.Turned_On;                           -- 12 return statement required of every function
>     end Is_On;                                        -- 13 Scope terminator for function
> end Machinery_1_3;                                    -- 14 End of all declarations for this package

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The code you just posted is directly copied from Ada Distilled.   The specification will compile just fine since it is only a specification.  You must compile the specification before the body.  They should be compiled separately.   

Richard Riehle, PhD    (Author of Ada Distilled)