Pascal interpreter written in Swift
Simple Swift interpreter for the Pascal language inspired by the Let’s Build A Simple Interpreter article series.
What is implemented
- standard types (integer, real, boolean, string)
- arithmetic expressions
- function calls
- procedure calls
- loops (for, repet until, while)
- logical conditions (if)
- standard Pascal functions (writeln, write, readln, read, random)
- one-dimensional arrays
You can see what the AST looks like in the unit tests or in the Playground where you can also use the
printTree() method on any AST to see its visual representation printed into the console.
The Semantic analyzer does static semantic checks on the Pascal program AST. It currently checks if all the used variables are declared beforehand and if there are any duplicate declarations. The result of semantic analysis is a Symbol table that holds all the symbols used by a Pascal program, currently built in types (Integer, Real, Boolean, String) and declared variable names.
- Check if a variable was declared with a known type (Integer, Real)
- Check if a variable was declared before usage
- Check if variable is not declared more than once
- Check if a procedure was declared
- Check if a procedure is called with the correct number of parameters
The Interpreter reads the AST representing the Pascal program from Parser and interprets it by walking the AST recursively. It can handle basic Pascal programs.
At the end of the Pascal program interpretation you can check the resulting memory state (see unit tests) or print it in the Playground using
Try it out
When you build the SPI project in the workspace you will get command line utility that can run any Pascal program given as argument, as shown in the GIF at the top of this README.
There is a Swift playground in the project where you can try out the lexer, parser and the interpreter. The Playground interprets then following Pascal program defining and calling a factorial function
program Main; var result: integer; function Factorial(number: Integer): Integer; begin if number > 1 then Factorial := number * Factorial(number-1) else Factorial := 1 end; begin writeln('Factorial'); result := Factorial(6); writeln(result) end.