ExpressionInterface

ExpressionInterface is a programming interface used to represent a node in an expression tree that can be evaluated, transformed, or serialized by software components. It provides a contract for operations that can be performed on expressions, independent of their concrete type. In many libraries, an expression can be a literal value, a variable reference, a binary operation, or a function call.

Core methods typically include: evaluate(context) which computes the expression’s value given a runtime context or bindings; toString() or render() for human-readable or serialized forms; and possibly accept(visitor) for the Visitor design pattern. Some designs also expose methods for optimization or compilation, such as simplify(), optimize(), or compile() to a target such as SQL or bytecode. The exact method set can vary by language and framework.

An implementation builds expression trees by composing simple expressions into more complex ones. Common concrete classes include LiteralExpression, VariableExpression, BinaryExpression, UnaryExpression, and FunctionExpression. The evaluate method traverses the tree and combines results according to operators and functions, consulting the provided context for variable values and function definitions.

Usage scenarios include rule engines, dynamic query builders, templating systems, mathematical calculators, and expression evaluators embedded in configuration systems. The interface supports modularity and reuse by enabling algorithms to operate on any expression type in a uniform way.

Design notes: ExpressionInterface is often used with the Composite pattern to treat individual expressions and composite expressions uniformly, and with the Visitor pattern to separate algorithms from the data structure. Immutability and thread-safety are common goals to avoid side effects during evaluation.

Related topics include abstract syntax trees, the interpreter pattern, the composite pattern, the visitor pattern, rule engines, and expression grammars.