In the early days of computing, language design was heavily influenced by the decision to use compilation or interpretation as a mode of execution. For example, some compiled languages require that programs must explicitly state the data-type of a variable at the time it is declared or first used. On the other hand, some languages take advantage of the dynamic aspects of interpretation to make such declarations unnecessary. For example, Smalltalk—which was designed to be interpreted at run-time—allows generic Objects to dynamically interact with each other.
Nowadays, the differences between the two styles of execution have largely been dealt with by more sophisticated designs. Most so-called interpreted languages use an intermediate representation, which combines both compilation and interpretation. In this case, a compiler may output some form of bytecode, which is then executed by a bytecode interpreter. Examples include Python, Java, Perl. Similarly, Ruby uses an abstract syntax tree as intermediate representation. The intermediate representation can be compiled once and for all, like Java, each time before execution like Perl or Ruby, or each time a change in the source is detected before execution like Python.
Interpreted languages still give programs certain extra flexibility over compiled languages. Features that are easier to implement in interpreters than in compilers include (but are not limited to)