A read–eval–print loop (REPL) is a simple, interactive computer programming environment. The term is most usually used to refer to a Lisp interactive environment, but can be applied to command line shells and similar environments for programming languages (see examples section, below). Synonyms include interactive toplevel and language shell.
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 read function accepts an expression from the user, and parses it into a data structure in memory. For instance, the user may enter the s-expression
(+ 1 2 3), which is parsed into a linked list containing four data elements.
- The eval function takes this internal data structure and evaluates it. In Lisp, evaluating an s-expression beginning with the name of a function means calling that function on the arguments that make up the rest of the expression. So the function
+is called on the arguments
1 2 3, yielding the result
- The print function takes the result yielded by eval, and prints it out to the user. If it is a complex expression, it may be pretty-printed to make it easier to understand. In this example, though, the number
6does not need much formatting to print.
A REPL can become an essential part of learning a new language as it gives quick feedback to the novice. Many tool-suites as well as programming languages use a REPL to allow algorithm exploration and debug, such as MATLAB, ROOT, SciPy and IPython.
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.
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. Typical functionality provided by a Lisp REPL includes:
- History of inputs and outputs.
- Variables are set for the input expressions and results. These variables are also available in the REPL. For example in Common Lisp * refers to the last result, ** and *** to the results before that.
- Levels of REPLs. In many Lisp systems if an error occurs during the reading, evaluation or printing of an expression, the system is not thrown back to the top-level with an error message. Instead a new REPL, one level deeper, is started in the error context. The user can then inspect the problem, fix it and continue - if possible. If an error occurs in such a debug REPL, another REPL, again a level deeper, is started. Often the REPL offers special debug commands.
- Error handling. The REPL provides restarts. These restarts can be used, when an error occurs to go back to a certain REPL level.
- Mouse sensitive input and output of data objects.
- Input editing and context specific completion over symbols, pathnames, class names and other objects.
- Help and documentation for commands.
- Variables to control the reader. For example, the variable *read-base* controls in which base numbers are read by default.
- Variables to control the printer. Example: maximum length or maximum depth of expressions to print.
- Additional command syntax. Some REPLs have commands that follow not the s-expression syntax, but often work with Lisp data as arguments.
- Graphical REPLs. Some Lisp REPLs (the CLIM Listener is an example) accept also graphical input and output.