8. psyche.interpreter

This module provides the Scheme Interpreter, the interactive Shell and the Scheme Environments.

exception SchemeException

This exception is raised by the Shell and the Interpreter whenever a Scheme expression calls the built-in (error...) procedure. Its accompanying value is a tuple containing the arguments to the original error call.

exception UndefinedException

This exception is raised by the Shell and the Interpreter whenever a Scheme command tries to reference an undefined variable. The referenced name can be obtained by calling the name method.

class Environment([parent])

The base class for Scheme environments. It provides dictionary access by implementing the magic __getitem__ and __setitem__ methods. If a key is not found, it is subsequently looked up in the parent environment. By default, the parent environment is None

class Interpreter([environment])

Instances of this class represent a Scheme interpreter using the specified environment. If environment is not specified, it defaults to an instance of SchemeEnvironment5.

class SchemeEnvironment5()

Instances of this class represent a Scheme Environment as specified by the (scheme-report-environment 5) call in [1].

class Shell()

Instances of the Shell can be used for interactive access to the interpreter. When possible, it provides a readline interface. Each Shell contains its own interpreter.

8.1 Environment Objects

Each Environment instance represents a single Scheme environment; its methods can be subdivided into two groups: methods providing dictionary-like access and methods providing typical Environment functionality.

Environments can contain both procedures and variables; the difference between them is irrelevant to an Environment.

__getitem__(key)

__setitem__(key, value)

These methods allow dictionary access with the subscripting operator [].

keys()

Returns a list of all variables and procedure names in this environment.

update(dict)

Updates the entries in this Environment with the entries specified in the dictionary dict. This is especially useful to add multiple user-defined procedures and variables at once.

extend()

Extends this Environment by returning a new Environment that has the current Environment as its parent. The interpreter uses this method to implement local scopes.

8.2 Interpreter Objects

Each Interpreter instance represents a single Scheme interpreter. Different Interpreters do not interfere with each other; procedures defined in one interpreter will not be visible in other interpreters.

USE_TAIL_RECURSION

Boolean value indicating whether or not the Interpreter should use tail recursion. By default, it has the value 1. If set to false, the interpreter will not use tail recursion. However, by not using tail recursion Scheme expressions will be subjected to the Python recursion limit.

On the other hand, setting this flag to false will result in slightly faster execution since Psyche will not have to analyze the parse tree to mark those expressions in tail context.

environment()

Returns the environment used by the interpreter.

eval(line)

Evaluates the string line and returns the object resulting from evaluating the Scheme expression specified in line. If the result of this expression is undefined, eval will return None.

This method will raise all kinds of exceptions if anything went wrong.

reset()

Resets the current environment to the environment given at initialization by removing all entries that resulted from evaluating Scheme code. More specifically, if any standard procedures (such as (map...)) were shadowed as the result of evaluating a Scheme expression, calling reset will remove the shadowing definition.

8.3 Shell Objects

Each Shell instance represents a single interactive Shell. Different Shells will not interfere with each other.

complete(text, n)

Returns the nth completion of text. Used for tab-completion if the readline interface is active. The default implementation completes keywords and elements from the interpreter's environment.

run()

Executes the read-eval-print loop until scheme_input returns (quit).

scheme_input()

Prompts on sys.stdout and reads from sys.stdin until an expression has been read with at least as many closing brackets as opening brackets. This expression is then returned as a joined line.