Gentle is a high-level programming language for compiler writers. It covers the whole spectrum of compiler construction, ranging from analysis over transformation to synthesis. Gentle provides a uniform notation for all tasks.
Gentle allows the user to define mutally recursive types by enumerating alternative structures
Expr = plus(Expr,Expr), minus(Expr,Expr), const(INT)Programs in Gentle are expressed by rules of the form
G : A B CThese rules may be interpreted as grammar rules ( G is constructed from A, B, and C), as logical statements ( G is true if A, B, and C are true), or in a procedural manner (to solve task G, solve subtasks A, B, and C).
Members of a rule may have parameters (an arrow separating input from output parameters). This results in attributed grammars
AddingExpression(-> plus(X1, X2)): AddingExpression(-> X1) "+" Primary(-> X2)or in transformation schemes that inductively follow the structure of terms
Eval(plus (X1, X2)->N1+N2): Eval(X1->N1) Eval(X2->N2). Eval(minus(X1, X2)->N1-N2): Eval(X1->N1) Eval(X2->N2). Eval(const(N) ->N).Unparsing may be expressed in a similar way
Code(plus(X1, X2) -> Reg2) : Code(X1 -> Reg1) Code(X2 -> Reg2) Emit("add", Reg1, Reg2).Such rules can be augmented by cost values. Then, rules are selected in such a way that the sum of the costs of all selected rules is optimal. This allows a nondeterministic specification that handles idioms of the target machine by rules with corresponding patterns.
Gentle is a strongly typed language that allows a tool to statically check the consistency of the specification. Traditional errors such as uninitialized variables can be detected at the earliest point.
Gentle has a declarative flavor. It also provides mutable global variables and tables to represent deferred information and cyclic structures. Gentle supports procedures written in other languages.
Gentle descends from CDL  and Prolog . An early version was described in .