| Home | Libraries | People | FAQ | More

### Glossary

callable transform

  A transform of the form `R(A0,A1,...)` (i.e., a function type) where `proto::is_callable<R>::value` is `true`. `R` is treated as a polymorphic function object and the arguments are treated as transforms that yield the arguments to the function object.

context

  In Proto, the term context refers to an object that can be passed, along with an expression to evaluate, to the `proto::eval()` function. The context determines how the expression is evaluated. All context structs define a nested `eval<>` template that, when instantiated with a node tag type (e.g., `proto::tag::plus`), is a binary polymorphic function object that accepts an expression of that type and the context object. In this way, contexts associate behaviors with expression nodes.

domain

  In Proto, the term domain refers to a type that associates expressions within that domain with a generator for that domain and optionally a grammar for the domain. Domains are used primarily to imbue expressions within that domain with additional members and to restrict Proto's operator overloads such that expressions not conforming to the domain's grammar are never created. Domains are empty structs that inherit from `proto::domain<>`.

domain-specific language

  A programming language that targets a particular problem space by providing programming idioms, abstractions and constructs that match the constructs within that problem space.

embedded domain-specific language

  A domain-specific language implemented as a library. The language in which the library is written is called the "host" language, and the language implemented by the library is called the "embedded" language.

expression

  In Proto, an expression is a heterogeneous tree where each node is either an instantiation of `boost::proto::expr<>`, `boost::proto::basic_expr<>` or some type that is an extension (via `boost::proto::extends<>` or `BOOST_PROTO_EXTENDS()`) of such an instantiation.

expression template

  A C++ technique using templates and operator overloading to cause expressions to build trees that represent the expression for lazy evaluation later, rather than evaluating the expression eagerly. Some C++ libraries use expression templates to build embedded domain-specific languages.

generator

  In Proto, a generator is a unary polymorphic function object that you specify when defining a domain. After constructing a new expression, Proto passes the expression to your domain's generator for further processing. Often, the generator wraps the expression in an extension wrapper that adds additional members to it.

grammar

  In Proto, a grammar is a type that describes a subset of Proto expression types. Expressions in a domain must conform to that domain's grammar. The `proto::matches<>` metafunction evaluates whether an expression type matches a grammar. Grammars are either primitives such as `proto::_`, composites such as `proto::plus<>`, control structures such as `proto::or_<>`, or some type derived from a grammar.

object transform

  A transform of the form `R(A0,A1,...)` (i.e., a function type) where `proto::is_callable<R>::value` is `false`. `R` is treated as the type of an object to construct and the arguments are treated as transforms that yield the parameters to the constructor.

polymorphic function object

  An instance of a class type with an overloaded function call operator and a nested `result_type` typedef or `result<>` template for calculating the return type of the function call operator.

primitive transform

  A type that defines a kind of polymorphic function object that takes three arguments: expression, state, and data. Primitive transforms can be used to compose callable transforms and object transforms.

sub-domain

  A sub-domain is a domain that declares another domain as its super-domain. Expressions in sub-domains can be combined with expressions in the super-domain, and the resulting expression is in the super-domain.

transform

  Transforms are used to manipulate expression trees. They come in three flavors: primitive transforms, callable transforms, or object transforms. A transform ``T`` can be made into a ternary polymorphic function object with `proto::when<>`, as in `proto::when<proto::_, `T`>`. Such a function object accepts expression, state, and data parameters, and computes a result from them.