Ada Reference Manual (Ada 202x Draft 25)Legal Information
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4.2.1 User-Defined Literals

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Using one or more of the aspects defined below, a type may be specified to allow the use of one or more kinds of literals as values of the type. 

Static Semantics

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The following type-related operational aspects (collectively known as user-defined literal aspects) may be specified for any type T:
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Integer_Literal

This aspect is specified by a function_name that statically denotes a function with a result type of T and one in parameter that is of type String and is not explictly aliased.
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Real_Literal
This aspect is specified by a function_name that statically denotes a function with a result type of T and one in parameter that is of type String and is not explictly aliased.
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String_Literal

This aspect is specified by a function_name that statically denotes a function with a result type of T and one in parameter that is of type Wide_Wide_String and is not explictly aliased.
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User-defined literal aspects are inherited according to the rules given in 13.1.
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When a numeric literal is interpreted as a value of a non-numeric type T or a string_literal is interpreted a value of a type T that is not a string type (see 4.2), it is equivalent to a call to the subprogram denoted by the corresponding aspect of T: the Integer_Literal aspect for an integer literal, the Real_Literal aspect for a real literal, and the String_Literal aspect for a string_literal. The actual parameter of this notional call is a string_literal having the textual representation of the original (numeric or string) literal.
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Such a literal is said to be a user-defined literal.

Legality Rules

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The Integer_Literal or Real_Literal aspect shall not be specified for a type T if the full view of T is a numeric type. The String_Literal aspect shall not be specified for a type T if the full view of T is a string type.
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For a nonabstract type, the function directly specified for a user-defined literal aspect shall not be abstract.
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For a tagged type with a partial view, a user-defined literal aspect shall not be directly specified on the full type.
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If a nonabstract tagged type inherits any user-defined literal aspect, then each inherited aspect shall be directly specified as a nonabstract function for the type unless the inherited aspect denotes a nonabstract function and the type is a null extension.
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In addition to the places where Legality Rules normally apply (see 12.3), these rules also apply in the private part of an instance of a generic unit.

Bounded (Run-Time) Errors

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It is a bounded error if the evaluation of a literal that has an expected type with a specified user-defined literal aspect propagates an exception. Either Program_Error or the exception propagated by the evaluation is raised at the point of use of the value of the literal. If it is recognized prior to run time that evaluation of such a literal will inevitably (if executed) result in such a bounded error, then this may be reported as an error prior to run time.

Examples

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subtype Roman_Character is Character
   with Static_Predicate =>
      Roman_Character in 'I' | 'V' | 'X' | 'L' | 'C' | 'D' | 'M';
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Max_Roman_Number : constant := 3_999;  -- MMMCMXCIX
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type Roman_Number is range 1 .. Max_Roman_Number
   with String_Literal => To_Roman_Number;
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function To_Roman_Number (S : String) return Roman_Number
   with Pre => S'Length > 0 and then
      (for all Char of S => Char in Roman_Character);
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function To_Roman_Number (S : String) return Roman_Number is
   (declare
      R : constant array (Integer range <>) of Integer :=
         (for D in S'Range => Roman_Digit'Enum_Rep
             (Roman_Digit'Value (''' & S(D) & '''))); -- See 3.5.2 and 13.4
    begin
      [for I in R'Range =>
         (if I < R'Last and then R(I) < R(I + 1) then -1 else 1) * R(I))]
            'Reduce("+", 0)
   );
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X : Roman_Number := "III" * "IV" * "XII"; -- 144 (that is, CXLIV)

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