Can .ads be compiled alone?

[moixa <moixa.dan@gmail.com>]

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?

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[3] http://stackoverflow.com/questions/9921794/error-when-compiling-spec-files

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[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

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[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