6. Scheme Procedures in Psyche

All procedures for Scheme described in R5RS[1] are implemented in Python in the module psyche.schemefct. Since Scheme identifiers are a superset of Python identifiers, some name mangling was necessary. Since the complete list of Scheme procedures is already covered in R5RS, this Chapter will focus on the name mangling.

6.1 Name Mangling

This Section describes the algorithm used for name mangling. In this discussion, ``Scheme procedure'' refers to the procedure from R5RS while ``Python function'' refers to the implementation of a Scheme procedure in psyche.schemefct.

Table 6.1 contains some examples.

Scheme procedures are all in lower case, with separate words divided by dashes. Python functions use mixed case, where the first word is lowercase and the first character of each following word is capitalized.

In Scheme, all operators are actually identifiers or parts of identifiers. The Python functions use the same naming convention as the module operators.

Scheme procedures implementing predicates all end with a question mark. Python functions use the standard Python naming scheme by prepending the name with is (and using appropriate capitalization).

Scheme procedures use the exclamation mark to specify state-changing operations. Python functions omit the exclamation mark.

Scheme procedures use -> for functions that convert one type to another. Python uses the word to (with appropriate capitalization).

reserved names
Sometimes, after applying the previous mangling steps, the resulting name is not appropriate in Python. It can be a keyword or the name of a builtin or the name of a well-known module. These words are suffixed with an underscore. So the Scheme procedures not, list and string are implemented by the Python functions not_, list_ and string_

Table 6.1: Some name mangling examples

Scheme procedure Python function
current-input-port currentInputPort
+ add
number? isNumber
string-set! stringSet
string->number stringToNumber
char-ci<=? isCharCiLe

6.2 Calling Scheme Procedures from Python

If there is need to call one of the Scheme procedures in Python, there are two possibilities: the first option is to do the name mangling yourself. This is not too difficult, but quite error-prone.

The alternative is the procedures variable in psyche.schemefct. This is a dictionary mapping all Scheme procedure names to the corresponding Python function.

While it is possible to modify the procedures dictionary, you are advised not to. This variable is used to initialize instances of SchemeEnvironment5, the initial environment for most Interpreters.

Example 6..1   Using number? in Python
from psyche import schemefct, interpreter

i = interpreter.Interpreter()
obj = i.eval("5")

result = schemefct.isNumber(obj)
# or
result = schemefct.procedures["number?"](obj)

Y. Duppen