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Special Forms

Special form expressions contain a special form as the operator. Special form expressions do not follow the same rules of evaluation as call expressions. Each special form has its own rules of evaluation -- that's what makes them special! Here's the Scheme Specification to reference the special forms we will cover in this class.

It is important to note that everything in Scheme is either an atomic or an expression, so although these special forms look and operate similarly to Python, they are evaluated differently.

Special forms like if, cond, and, or in Python direct the control flow of a program and allow you to evaluate specific expressions under some condition. In Scheme, however, these special forms are expressions that take in a set amount of parameters and return some value based on the condition passed in.

Built-in Procedures

To be able to call built-in procedures, such as +, you need to complete the BuiltinProcedure case within the scheme_apply function in scheme_eval_apply.py. Built-in procedures are applied by calling a corresponding Python function that implements the procedure.

To see a list of all Scheme built-in procedures used in the project, look in the scheme_builtins.py file. Any function decorated with @builtin will be added to the globally-defined BUILTINS list.

A BuiltinProcedure has two instance attributes:

  • py_func: the Python function that implements the built-in Scheme procedure.
  • need_env: a Boolean flag that indicates whether or not this built-in procedure will need the current environment to be passed in as the last argument. The environment is required, for instance, to implement the built-in eval procedure.

scheme_apply takes the procedure object, a list of argument values, and the current environment. args is a Scheme list represented as a Pair object or nil.