Runtime

rascal-0.34.0

Synopsis

The runtime representation of Lisp programs and data.

Description

There are several aspects of the runtime representation of Lisp programs and Lisp data that have to be described:

• The representation of values (see Lval below).
• The handling of the scope of variables (see Scope, Env, makeEnv and find below).
• The way the interpreter returns its results (see Result below).

module demo::lang::Lisra::Runtime

import Prelude;

data Lval      ❶  
     = Integer(int n)   
     | Atom(str name)
     | List(list[Lval] elms)
     | Closure(Result(list[Lval] args, Env env))
     ;
         
alias Scope  = map[Lval,Lval];      ❷  
alias Env    = list[Scope];

public Env emptyEnv = [()];

Env makeEnv(list[Lval] vars, list[Lval] values, Env outer) =      ❸  
   [(vars[i] : values[i] | i <- index(vars))] + outer;

int find(Lval sym, Env e){      ❹  
   for(n <- index(e))
       if(e[n][sym]?)
          return n;
   return -1;
}

public Lval TRUE  = Atom("#t");      ❺  
public Lval FALSE = Atom("#f");

alias Result = tuple[Lval val, Env env];      ❻  

• ❶ The data type Lval takes care of the representation of Lisp values. It covers integers, atoms, lists and closures (the representation of a function and the context in which it will be executed).

• ❷ A Scope describes the binding of several related variables to their value. Since scopes may be nested, an environment (Env) consisted of a list of scope. The most inner scope is at the start of the list and the most global one at the end.

• ❸ Creating a new scope is done by makeEnv which takes a list of variables (represented by Lvals, in most cases this will be an atom like Atom("X")), a list of values and creates a new scope in front of the current environment.

• ❹ The function find tries to locate the scope in which a name was previously defined. It searches the nested scopes inside-out and returns the index in the given environment of the scope in which the name is defined, or -1 if it is not found.

• ❺ We define useful constants for true and false (the atoms #t and #f, respectively).

• ❻ Finally, we define Result as a tuple of an Lval and an Env. Each step during interpretation will thus return the value it computed and a possibly modified environment.