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Simple Swift interpreter for the Pascal language inspired by theLet’s Build A Simple Interpreter article series.

• standard types (integer, real, boolean, string)
• arithmetic expressions
• function calls
• procedure calls
• recursion
• loops (for, repet until, while)
• logical conditions (if)
• standard Pascal functions (writeln, write, readln, read, random)
• one-dimensional arrays

There are a few sample Pascal programs in theExamples directory, like a simplenumber guessing game and afactorial computation.

TheLexer reads the Pascal program asString (a sequence of characters) and converts it into a sequence ofTokens. You can see the result by trying it out in the Playground or on theunit tests.

TheParser reads the sequence of tokens produced by the Lexer and builds anAbstract Syntax Tree representation (AST for short) of the Pascal program according to thegrammar.

You can see what the AST looks like in theunit tests or in the Playground where you can also use theprintTree() method on any AST to see its visual representation printed into the console.

TheSemantic 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 aSymbol table that holds all the symbols used by a Pascal program, currently built in types (Integer, Real, Boolean, String) and declared variable names.

Implemented checks

• 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

TheInterpreter 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 (seeunit tests) or print it in the Playground usingprintState().

When you build theSPI 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;functionFactorial(number: Integer): Integer;beginif number >1then    Factorial := number * Factorial(number-1)else    Factorial :=1end;beginwriteln('Factorial');result := Factorial(6);writeln(result)end.

Igor Kulman -igor@kulman.sk

This project is licensed under the MIT License - see theLICENSE file for details