Interpretation

The simplest way to execute a program is interpretation. Interpretation is done by a program called an interpreter, which takes the abstract syntax tree of a program and executes it by inspecting the syntax tree to see what needs to be done. This is similar to how a human evaluates a mathematical expression: We insert the values of variables in the expression and evaluate it bit by bit, starting with the innermost parentheses and moving out until we have the result of the expression. We can then repeat the process with other values for the variables. There are some differences, however. Where a human being will copy the text of the formula with variables replaced by values, and then write a sequence of more and more reduced copies of a formula until it is reduced to a single value, an interpreter will keep the formula (or, rather, the abstract syntax tree of an expression) unchanged and use a symbol table to keep track of the values of variables. Instead of reducing a formula, the interpreter is a function that takes an abstract syntax tree and a symbol table as arguments and returns the value of the expression represented by the abstract syntax tree. The function can call itself recursively on parts of the abstract syntax tree to find the values of subexpressions, and when it evaluates a variable, it can look its value up in the symbol table. This process can be extended to also handle statements and declarations, but the basic idea is the same: A function takes the abstract syntax tree of the program and, possibly, some extra information about the context (such as a symbol table or the input to the program) and returns the output of the program.