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Right now it represented in two ways - global AST transformations and local AST transformations. 9 | 10 | While local transformations are very useful, they are (obviously) limited to the scope where they are applied. It covers most of the cases, but sometimes the scope is global, so we have to use global AST transformations. Some examples of them: 11 | - Spock 12 | - GORM 13 | - Groovy's Grab 14 | - Griffon 15 | - MacroGroovy (available as `macro` method in groovy-core since 2.5) 16 | 17 | During my work on MacroGroovy I discovered the pattern which can be reused - replace method call with AST it produces as a global AST transformation. It's called "Syntactic macros", and it's a common thing for the languages like Lisp, Scala, Rust, Haxe and others. Consider the following: 18 | ```java 19 | @Macro // <-- macro method aka magic! 20 | static Expression warn(MacroContext ctx, Expression exp, Expression msg) { 21 | // compile time property check! 22 | if ("development" !== System.getProperty("ENV")) { 23 | return new EmptyExpression() 24 | } 25 | 26 | String src = toCodeSource(ctx); 27 | return macro { 28 | !$v{exp} && println($v{src} + ": " + $v{msg}) 29 | } 30 | } 31 | ``` 32 | 33 | And the usage: 34 | ```java 35 | int age = 10; 36 | 37 | warn(age >= 18, "User is under 18"); 38 | ``` 39 | 40 | It will print (in runtime) a message **only if `ENV` system property was equal to "development" during the compilation**. Not the runtime, but compilation time. So, if you compile with "-DENV=development", the code will be transformed to: 41 | ```java 42 | int age = 10; 43 | 44 | !(age > 18) && println("myFile.groovy:3" + ": " + "User is under 18") 45 | ``` 46 | 47 | But the same code compiled without `ENV=development` it will be equal to: 48 | ```java 49 | int age = 10; 50 | ``` 51 | 52 | As you can see, macro methods are being executed at compile time and they must return an AST expression, so compiler will replace such call with it. 53 | 54 | Another nice thing that you can get the information about the place where macro code was injected by accessing `MacroContext` (again, at compile time!). In this example we use `toCodeSource(context)` to get the position in source code where warning was called. 55 | 56 | --- 57 | Good parts so far: 58 | - **Compile-time** meta programming 59 | - Access to the place where macro method was called 60 | - Almost **as simple as method call**, but with all the power of compile-time meta programming 61 | 62 | # Usage for library developers 63 | Macro methods are useful if you develop a library as well. For instance, imagine you're writing ORM library, let's call it "GroORM" :) 64 | 65 | We want to provide a method to perform compile-time, type safe queries: 66 | ```groovy 67 | def targetAudience = User.sqlQuery { 68 | select(id, name as username) where age > 18 && age < 25 orderBy age 69 | } 70 | ``` 71 | 72 | This can be implemented with Macro method: 73 | ```java 74 | @Macro 75 | static Expression warn(MacroContext ctx, ClosureExpression block) { 76 | def modelClassExp = context.call.objectExpression; 77 | 78 | return convertASTToSQLQuery(block); 79 | } 80 | ``` 81 | 82 | And, after compilation, it will look something like this: 83 | ```java 84 | def targetAudience = GroORM.executeSQL(User, ''' 85 | SELECT 86 | id, name as username 87 | WHERE 88 | age > 18 and age < 25 89 | ORDER BY 90 | age 91 | ''') 92 | ``` 93 | It can also check that User class has `id`, `name`, `password` and `age` fields at compile time. 94 | 95 | Even more - it can check that age is number type, for instance. 96 | 97 | The result's type will be `List` because GroORM can parse SQL expression at compile time and make some assumptions about the result. 98 | 99 | --- 100 | To sum up: 101 | - Libraries can implement powerful, type-safe, compile-time conversions without manually dealing with Global AST transformations. 102 | - Runtime performance is great because transformations are being done in compile time. 103 | - Thanks to Groovy's flexible syntax, one can implement really powerful expressions like an example above - `name as username` is something common for SQL. We have `as` operator in Groovy, so, until "Semantic analysis" phase, it's just a CastExpression! 104 | 105 | # Current status 106 | Currently proposal is implemented as a 3rd party library: https://github.com/bsideup/groovy-macro-methods 107 | 108 | This is a global AST transformation and all macro methods share the same transformation (vs "Global transformation per use case") 109 | 110 | For each static method marked with `@Macro` annotation it will create an internal Groovy Extension Method with the same name and signature: 111 | ```java 112 | (MacroContext, ...argExpressions) 113 | ``` 114 | 115 | So, if we have 116 | ```java 117 | @Macro 118 | static Expression mySuperMethod(MacroContext ctx, ConstantExpression constExp) 119 | ``` 120 | 121 | we will get: 122 | ```java 123 | mySuperMethod("Hello") // match 124 | mySuperMethod(123) // match 125 | 126 | mySuperMethod(123, "Hello") // no match, too many arguments 127 | mySuperMethod(prefix + " World!") // no match, argument is CallExpression 128 | mySuperMethod {} // no match, argument is ClosureExpression 129 | mySuperMethod() // no match, arguments are empty 130 | ``` 131 | 132 | # Try it yourself 133 | You can try Macro methods right now with the latest Groovy release. They are implemented as a library for now. 134 | 135 | For instance, here is an example of Pattern Matching for Groovy implemented with Macro methods: 136 | https://github.com/bsideup/groovy-pattern-match 137 | 138 | It depends on `groovy-macro-methods` available in JCenter: 139 | https://github.com/bsideup/groovy-pattern-match/blob/d7c4c5494238b84b7685ce5fc433c8676416f1dc/build.gradle#L24-L24 140 | ```gradle 141 | dependencies { 142 | compile 'ru.trylogic.groovy.macro:groovy-macro-methods:0.2.0' 143 | } 144 | ``` 145 | 146 | It implements macro method `match`: 147 | 148 | https://github.com/bsideup/groovy-pattern-match/blob/d7c4c5494238b84b7685ce5fc433c8676416f1dc/src/main/java/ru/trylogic/groovy/pattern/PatternMatchingMacroMethods.java#L38-L41 149 | 150 | So you can write Groovy code with compile-time pattern matching: 151 | ```java 152 | def fact(num) { 153 | return match(num) { 154 | when String then fact(num.toInteger()) 155 | when 0 or 1 then 1 156 | when 2 then 2 157 | orElse it * fact(it - 1) 158 | } 159 | } 160 | 161 | assert fact("5") == 120 162 | ``` 163 | 164 | (For more examples check the tests: https://github.com/bsideup/groovy-pattern-match/blob/e2c2b0472b7d078a8b299085705584cfea827405/src/test/groovy/ru/trylogic/groovy/pattern/PatternMatchingMacroMethodsTest.groovy) 165 | 166 | # Open questions 167 | - Syntax: https://github.com/bsideup/groovy-macro-methods-proposal/issues/1 168 | - Global nature: https://github.com/bsideup/groovy-macro-methods-proposal/issues/2 --------------------------------------------------------------------------------