Read–eval–print loop

A read–eval–print loop (REPL), also known as an interactive toplevel or language shell, is a simple, interactive computer programming environment that takes single user inputs (i.e. single expressions), evaluates them, and returns the result to the user; a program written in a REPL environment is executed piecewise. The term is most usually used to refer to programming interfaces similar to the classic Lisp machine interactive environment. Common examples include command line shells and similar environments for programming languages.

Overview

In a REPL, the user enters one or more expressions (rather than an entire compilation unit) and the REPL evaluates them and display the results. The name read–eval–print loop comes from the names of the Lisp primitive functions which implement this functionality:

The development environment then returns to the read state, creating a loop, which terminates when the program is closed.

REPLs facilitate exploratory programming and debugging because the read–eval–print loop is usually much faster than the classic edit-compile-run-debug cycle.

Because the print function outputs in the same textual format that the read function uses for input, most results are printed in a form that could (if it is useful) be copied and pasted back into the REPL. However, it's sometimes necessary to print representations of elements that can't sensibly be read back in, such as a socket handle or a complex class instance. In these cases, there must exist a syntax for unreadable objects. In Python, it is the <__module__.class instance> notation, and in Common Lisp, the #<whatever> form. The REPL of CLIM, SLIME, and the Symbolics Lisp Machine can also read back unreadable objects. They record for each output which object was printed. Later when the code is read back, the object will be retrieved from the printed output.

REPLs can be created to support any language. REPL support for compiled languages is usually achieved by implementing an interpreter on top of a virtual machine which provides an interface to the compiler. Examples of REPLs for compiled languages include CINT (and its successor Cling), Ch, and BeanShell

Uses

Nearly all UNIX shells are REPLs for Turing complete languages. As a shell, a REPL environment allows users to access relevant features of an operating system in addition to providing access to programming capabilities.

The most common use for REPLs outside of operating system shells is for instantaneous prototyping. Other uses include mathematical calculation, creating documents that integrate scientific analysis (e.g. IPython), interactive software maintenance, benchmarking, and algorithm exploration.

A REPL can become an essential part of learning a new language as it gives quick feedback to the novice.

Implementation

To implement a Lisp REPL, it is necessary only to implement these three functions and an infinite-loop function. (Naturally, the implementation of eval will be complicated, since it must also implement all the primitive functions like car and + and special operators like if.) This done, a basic REPL itself is but a single line of code: 

(loop (print (eval (read))))

One possible implementation of eval is as a recursive interpreter that acts on the abstract syntax tree created by read. Another possibility is to compile the syntax tree into machine code and execute it.

Real REPL implementations in Lisp are often much more complicated.

Functionality

Typical functionality provided by a Lisp REPL includes:

External links