Fixing nested function parsing in custom programming language
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CodeParser_Dev
Posted on August 5, 2024 • Advanced
🔄 Nested function parsing causing infinite loops
I’m building a custom programming language interpreter in Scratch, but I’m having major issues with nested function calls. When I try to evaluate expressions like:
sqrt(abs(-7)+sqrt(4))max(min(5,3), abs(-2))- Any function with nested parentheses
The parser gets stuck in infinite loops and never returns a result. I think the issue is with how I’m handling the recursive parsing, but I can’t figure out what’s going wrong. 😵
Has anyone built something similar? How do you properly parse nested function calls without getting stuck?
LanguageParser_Expert
Replied 4 hours later • ⭐ Best Answer
@CodeParser_Dev I’ve built several interpreters in Scratch! The infinite loop issue is very common with recursive parsing. Let me show you a robust solution:
🔍 Step 1: Understanding the Problem
The issue is usually caused by:
- Not properly extracting inner expressions
- Missing base cases in recursion
- Incorrect parentheses matching
- Stack overflow from too deep recursion
🛠️ Step 2: Proper Expression Tokenizer
First, let’s build a tokenizer that breaks down the expression:
// Tokenize expression into manageable parts define tokenize (expression) delete all of [tokens v] set [i v] to [1] set [current token v] to [] repeat (length of (expression)) set [char v] to (letter (i) of (expression)) if <<(char) = [(]> or <(char) = [)]>> then if <(length of (current token)) > [0]> then add (current token) to [tokens v] set [current token v] to [] end add (char) to [tokens v] else if <<(char) = [,]> or <(char) = [ ]>> then if <(length of (current token)) > [0]> then add (current token) to [tokens v] set [current token v] to [] end else set [current token v] to (join (current token) (char)) end end change [i v] by [1] end if <(length of (current token)) > [0]> then add (current token) to [tokens v] end
🎯 Step 3: Recursive Descent Parser
Here’s the core parsing logic that handles nested functions properly:
// Main evaluation function define evaluate (expression) set [recursion depth v] to ((recursion depth) + [1]) if <(recursion depth) > [50]> then say [Error: Maximum recursion depth exceeded] for (2) seconds set [recursion depth v] to ((recursion depth) - [1]) stop [this script v] end // Remove spaces set [clean expr v] to (replace [ ] with [] in (expression)) // Check if it's just a number if <(is number? (clean expr)) = [true]> then set [result v] to (clean expr) set [recursion depth v] to ((recursion depth) - [1]) else // Parse function call set [result v] to (parse function call (clean expr)) set [recursion depth v] to ((recursion depth) - [1]) end
🔧 Step 4: Function Call Parser
This handles the actual function parsing with proper nesting:
define parse function call (expr) set [func name v] to [] set [paren count v] to [0] set [start pos v] to [0] set [i v] to [1] // Find function name repeat until <<(letter (i) of (expr)) = [(]> or <(i) > (length of (expr))>> set [func name v] to (join (func name) (letter (i) of (expr))) change [i v] by [1] end if <(i) > (length of (expr))> then // No function, just return the expression set [parse result v] to (expr) else // Extract arguments set [start pos v] to ((i) + [1]) // Skip opening parenthesis set [paren count v] to [1] change [i v] by [1] repeat until <<(paren count) = [0]> or <(i) > (length of (expr))>> if <(letter (i) of (expr)) = [(]> then change [paren count v] by [1] end if <(letter (i) of (expr)) = [)]> then change [paren count v] by [-1] end change [i v] by [1] end // Extract inner expression (without outer parentheses) set [inner expr v] to (letters (start pos) to ((i) - [2]) of (expr)) // Parse arguments parse arguments (inner expr) // Execute function execute function (func name) end
⚙️ Step 5: Argument Parser
This correctly splits arguments while respecting nested parentheses:
define parse arguments (args string) delete all of [arguments v] if <(length of (args string)) = [0]> then stop [this script v] end set [current arg v] to [] set [paren depth v] to [0] set [i v] to [1] repeat (length of (args string)) set [char v] to (letter (i) of (args string)) if <(char) = [(]> then change [paren depth v] by [1] set [current arg v] to (join (current arg) (char)) else if <(char) = [)]> then change [paren depth v] by [-1] set [current arg v] to (join (current arg) (char)) else if <<(char) = [,]> and <(paren depth) = [0]>> then // End of current argument add (evaluate (current arg)) to [arguments v] set [current arg v] to [] else set [current arg v] to (join (current arg) (char)) end end end change [i v] by [1] end // Add the last argument if <(length of (current arg)) > [0]> then add (evaluate (current arg)) to [arguments v] end
🎲 Step 6: Function Execution
Finally, execute the actual mathematical functions:
define execute function (name) if <(name) = [sqrt]> then if <(length of [arguments v]) = [1]> then set [parse result v] to ([sqrt v] of (item [1] of [arguments v])) else set [parse result v] to [Error: sqrt requires 1 argument] end else if <(name) = [abs]> then if <(length of [arguments v]) = [1]> then set [parse result v] to ([abs v] of (item [1] of [arguments v])) else set [parse result v] to [Error: abs requires 1 argument] end else if <(name) = [max]> then if <(length of [arguments v]) = [2]> then if <(item [1] of [arguments v]) > (item [2] of [arguments v])> then set [parse result v] to (item [1] of [arguments v]) else set [parse result v] to (item [2] of [arguments v]) end else set [parse result v] to [Error: max requires 2 arguments] end else if <(name) = [min]> then if <(length of [arguments v]) = [2]> then if <(item [1] of [arguments v]) < (item [2] of [arguments v])> then set [parse result v] to (item [1] of [arguments v]) else set [parse result v] to (item [2] of [arguments v]) end else set [parse result v] to [Error: min requires 2 arguments] end else set [parse result v] to (join [Error: Unknown function ] (name)) end end end end
🐛 Step 7: Debug Helper
Add this debugging system to track what’s happening:
define debug log (message) add (join (join [Depth ] (recursion depth)) (join [: ] (message))) to [debug log v] if <(length of [debug log v]) > [20]> then delete [1] of [debug log v] end when [d v] key pressed show list [debug log v] when [c v] key pressed hide list [debug log v] delete all of [debug log v]
🧪 Step 8: Test Cases
Test your parser with these examples:
when [1 v] key pressed set [test expr v] to [sqrt(4)] say (join [Result: ] (evaluate (test expr))) for (3) seconds when [2 v] key pressed set [test expr v] to [abs(-7)] say (join [Result: ] (evaluate (test expr))) for (3) seconds when [3 v] key pressed set [test expr v] to [sqrt(abs(-7)+sqrt(4))] say (join [Result: ] (evaluate (test expr))) for (3) seconds when [4 v] key pressed set [test expr v] to [max(min(5,3),abs(-2))] say (join [Result: ] (evaluate (test expr))) for (3) seconds
This approach should eliminate the infinite loops by properly handling recursion depth and correctly parsing nested expressions! 🎉
CodeParser_Dev
Replied 2 hours later
@LanguageParser_Expert This is absolutely incredible! 🤯
The recursion depth check was exactly what I was missing! My parser was getting stuck because it kept calling itself infinitely. Your solution with proper parentheses matching and argument parsing works perfectly.
I tested sqrt(abs(-7)+sqrt(4)) and it correctly returns 3! The debug system is super helpful too for understanding what’s happening at each step.
Thank you so much for this comprehensive solution! 🙏
Vibelf_Community
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Fantastic work on the parser implementation! For those interested in creating even more sophisticated language interpreters, our community can help you implement:
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