[−][src]Crate duplicate
This crate provides the duplicate attribute macro for
code duplication with substitution.
Usage
Say you have a trait with a method is_max that should return true if the
value of the object is the maximum allowed and false otherwise:
trait IsMax { fn is_max(&self) -> bool; }
You would like to implement this trait for the three integer types u8,
u16, and u32:
impl IsMax for u8 { fn is_max(&self) -> bool { *self == 255 } } impl IsMax for u16 { fn is_max(&self) -> bool { *self == 65_535 } } impl IsMax for u32 { fn is_max(&self) -> bool { *self == 4_294_967_295 } }
This is a lot of repetition. Only the type and the maximum value are
actually different between the three implementations. This might not be much
in our case, but imagine doing this for all the integer types (10, as of the
last count.) We can use the duplicate attribute to avoid repeating
ourselves:
use duplicate::duplicate; #[duplicate( int_type max_value; [ u8 ] [ 255 ]; [ u16 ] [ 65_535 ]; [ u32 ] [ 4_294_967_295 ]; )] impl IsMax for int_type { fn is_max(&self) -> bool { *self == max_value } } assert!(!42u8.is_max()); assert!(!42u16.is_max()); assert!(!42u32.is_max());
The above code will expand to the three implementations before it.
The attribute invocation specifies that the following item should be
substituted by three duplicates of itself. Additionally, each occurrence of
the identifier int_type in the first duplicate should be replaced by u8,
in the second duplicate by u16, and in the last by u32. Likewise, each
occurrence of max_value should be replaced by 255, 65_535, and
4_294_967_295 in the first, second, and third duplicates respectively.
int_type and max_value are called substitution identifiers, while [ u8 ], [ u16 ], and [ u32 ] are each substitutions for int_type. The
number of duplicates made is equal to the number of substitutions the
substitution identifiers have---all identifiers must have the same number of
substitutions. Substitution identifiers must be valid Rust identifiers.
The code inside substitutions can be arbitrary, as long as the expanded code
is valid. Additionally, any "bracket" type is valid; we could have used ()
or {} anywhere [] is used in these examples.
Verbose Syntax
The syntax used in the previous examples is the short syntax.
duplicate also accepts a verbose syntax that is less concise, but more
powerful. Using the verbose syntax, the above usage looks like this:
use duplicate::duplicate; #[duplicate( [ int_type [ u8 ] max_value [ 255 ] ] [ int_type [ u16 ] max_value [ 65_535 ] ] [ int_type [ u32 ] max_value [ 4_294_967_295 ] ] )] impl IsMax for int_type { fn is_max(&self) -> bool { *self == max_value } }
The verbose syntax is centered around the substitution group, which then includes a set of identifier and substitution pairs. Here is an annotated version of the same code:
#[duplicate( [ //-+ int_type [ u8 ] // | Substitution group 1 max_value [ 255 ] // | // ^^^^^^^^^ ^^^^^^^ substitution | // | | // substitution identifier | ] //-+ [ //-+ int_type [ u16 ] // | Substitution group 2 max_value [ 65_535 ] // | ] //-+ [ //-+ max_value [ 4_294_967_295 ] // | Substitution group 3 int_type [ u32 ] // | ] //-+ )]
Note that in each substitution group every identifier must have exactly one
substitution. Any number of groups can be given with each translating to one
duplicate. All the groups must have the exact same identifiers, though the
order in which they arrive in each group is not important. For example, in
the annotated example, the third group has the max_value identifier before
int_type without having any effect on the expanded code.
The short syntax's substitution grouping is based on the order of the substitutions for each identifier. We can annotate the short version of our example to highlight this:
#[duplicate( int_type max_value; [ u8 ] [ 255 ]; // Group 1 [ u16 ] [ 65_535 ]; // Group 2 [ u32 ] [ 4_294_967_295 ];// Group 3 )]
The verbose syntax is not very concise but it some advantages over the shorter syntax:
- Using many identifiers and long substitutions can quickly become unwieldy in the short syntax. The verbose syntax deals better with both as it will grow horizontally instead of vertically.
- It offers something the short syntax doesn't: nested invocation.
Nested Invocation
Imagine we have the following trait with the method is_negative that
should return true if the value of the object is negative and false
otherwise:
trait IsNegative { fn is_negative(&self) -> bool; }
We want to implement this for the six integer types u8, u16, u32,
i8, i16, and i32. For the first three types, which are all unsigned,
the implementation of this trait should trivially return false as they
can't be negative. However, for the remaining, signed types their
implementations is identical (checking whether they are less than 0), but,
of course, different from the first three:
impl IsNegative for u8 { fn is_negative(&self) -> bool { false } } impl IsNegative for u16 { fn is_negative(&self) -> bool { false } } impl IsNegative for u32 { fn is_negative(&self) -> bool { false } } impl IsNegative for i8 { fn is_negative(&self) -> bool { *self < 0 } } impl IsNegative for i16 { fn is_negative(&self) -> bool { *self < 0 } } impl IsNegative for i32 { fn is_negative(&self) -> bool { *self < 0 } }
Notice how the code repetition is split over 2 axes: 1) They all implement the same trait 2) the method implementations of the first 3 are identical to each other but different to the next 3, which are also mutually identical. To implement this using only the syntax we have already seen, we could do something like this:
#[duplicate( [ int_type [ u8 ] implementation [ false ] ] [ int_type [ u16 ] implementation [ false ] ] [ int_type [ u32 ] implementation [ false ] ] [ int_type [ i8 ] implementation [ *self < 0 ] ] [ int_type [ i16 ] implementation [ *self < 0 ] ] [ int_type [ i32 ] implementation [ *self < 0 ] ] )] impl IsNegative for int_type { fn is_negative(&self) -> bool { implementation } } assert!(!42u8.is_negative()); assert!(!42u16.is_negative()); assert!(!42u32.is_negative()); assert!(!42i8.is_negative()); assert!(!42i16.is_negative()); assert!(!42i32.is_negative());
However ironically, we here had to repeat ourselves in the macro invocation
instead of the code: we needed to repeat the implementations [ false ] and
[ *self < 0 ] three times each. Using verbose syntax we can utilize
nested invocation to remove the last bit of repetition:
#[duplicate(
#[
int_type_nested; [u8]; [u16]; [u32]
][
[
int_type [ int_type_nested ]
implementation [ false ]
]
]
#[
int_type_nested; [i8]; [i16]; [i32]
][
[
int_type [ int_type_nested ]
implementation [ *self < 0 ]
]
]
)]
impl IsNegative for int_type {
fn is_negative(&self) -> bool {
implementation
}
}
assert!(!42u8.is_negative());
assert!(!42u16.is_negative());
assert!(!42u32.is_negative());
assert!(!42i8.is_negative());
assert!(!42i16.is_negative());
assert!(!42i32.is_negative());We use # to invoke the macro inside itself, producing duplicates
of the code inside the following [], {}, or ().
In our example, we have 2 invocations that each produce 3 groups, inserting
the correct implementation for their signed or unsigned types.
The above nested invocation is equivalent to the previous, non-nested
invocation, and actually expands to it as an intermediate step before
expanding the outer-most invocation.
It's important to notice that the nested invocation doesn't know it
isn't the outer-most invocation and therefore doesn't discriminate between
identifiers. We had to use a different identifier in the nested invocations
(int_type_nested) than in the code (int_type), because otherwise the
nested invocation would substitute the substitution identifier, too, instead
of only substituting in the nested invocation's substitute.
Nested invocation is only possible when using verbose syntax. Additionally, the nested invocations must produce verbose syntax of their parent invocation. However, each nested invocation's private syntax is free to use the short version. Notice in our above example, the nested invocations use short syntax but produce verbose syntax for the outer-most invocation.
There is no limit on the depth of nesting, however, as might be clear from our example, it can get complicated to read. Additionally, the syntax used in any invocation that includes a nested invocation must be verbose.
Lastly, we should note that we can have nested invocations interleaved with
normal substution groups. For example, say we want to implement IsNegative
for i8, but don't want the same for i16 and i32. We could do the
following:
#[duplicate(
#[ // -+
int_type_nested; [u8]; [u16]; [u32] // |
][ // |
[ // | Nested invocation producing 3
int_type [ int_type_nested ] // | substitution groups
implementation [ false ] // |
] // |
] // -+
[ // -+
int_type [ i8 ] // | Substitution group 4
implementation [ *self < 0 ] // |
] // -+
)]
impl IsNegative for int_type {
fn is_negative(&self) -> bool {
implementation
}
}
Disclaimer
This crate does not try to justify or condone the usage of code duplication instead of proper abstractions. This macro should only be used where it is not possible to reduce code duplication through other means, or where it simply is not worth it.
As an example, libraries that have two or more structs/traits with similar APIs might use this macro to test them without having to copy-paste test cases and manually make the needed edits.
Attribute Macros
| duplicate | Duplicates and substitutes given identifiers for different code in each duplicate. |