uCalc API Version: 2.1.3-preview.2 Released: 6/17/2026

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uCalc API Preview Release Notice:The documentation describes the intended behavior of the API. The current preview build contains incomplete features, unoptimized performance, and is subject to breaking changes.

Project: Developing a Simple LISP Interpreter

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A step-by-step tutorial on building a simple LISP interpreter using uCalc's ExpressionTransformer.

Remarks

💡 Project: Building a Simple LISP Interpreter

This project is a masterclass in uCalc's dynamic syntax capabilities. We will build a simple, functional interpreter for a LISP-like language. LISP (LISt Processing) is famous for its distinctive parenthesized prefix notation, also known as S-expressions (Symbolic Expressions).

This project showcases how uCalc's Transformer can transpile one language's syntax into another on the fly using a set of declarative, recursive rules.

The Goal: Our LISP Syntax

We want our uCalc engine to understand expressions like this:

  • (+ 1 2)
  • (- 10 5)
  • (* 2 3 4) (variadic)
  • (/ 100 5 2) (variadic)
  • (+ 1 (* 2 3)) (nested)

These will be transformed into standard uCalc expressions (1 + 2, 10 - 5, etc.) and evaluated.

The Strategy: A Declarative, Multi-Rule Approach

A simple find-and-replace rule isn't enough because we need to handle nested and variadic expressions. The solution is to create a set of transformation rules that work together to recursively simplify the LISP expression from the inside out until only a single value remains.

This requires three key uCalc features:

  1. RewindOnChange(true): After a rule makes a replacement, the transformer re-scans the text, allowing other rules (or the same rule) to be applied to the result. This is the engine for our recursion.
  2. {@@Eval}: This directive evaluates a captured string as an expression. We'll use it to perform the arithmetic for each sub-expression.
  3. Immediate Transform (%): This variable modifier forces the transformer to evaluate nested patterns first, which is essential for our inside-out evaluation strategy.

Step 1: The Base Case - Binary Operations

The simplest rule handles a binary operation. It finds an operator followed by two operands, evaluates them, and replaces the entire S-expression with the result.

t.FromTo("({op:1} {a:1} {b:1})", "{@@Eval: a + op + b}");

This rule transforms (+ 10 20) into the result 30.

Step 2: The Recursive Step - Variadic Operations

To handle more than two operands, we define a rule that reduces the list one step at a time. It finds an operator and at least three operands, evaluates the first two, and puts the result back at the front of the list.

t.FromTo("({op:1} {a:1} {b:1} {more})", "({op} {@@Eval: a + op + b} {more})");

With RewindOnChange(true), this rule will be applied repeatedly:

  1. (* 2 3 4) becomes (* 6 4)
  2. The transformer rewinds and finds (* 6 4), which is then evaluated by the base case rule to 24.

Step 3: Inside-Out Evaluation - Nested Expressions

To handle nested expressions like (/ 100 (+ 5 5)), we need to evaluate the inner part first. The immediate transform modifier (%) on a variable forces this behavior.

t.FromTo("({op:1} {a:1} {expr%: ({exp})})", "({op} {a} {expr}");

This rule looks for an expression where the second operand is itself a parenthesized expression. The % on {expr%} tells the transformer: "Stop and fully evaluate the inner expression ({exp}) first, then substitute its result back here." RewindOnChange then allows the simplified outer expression to be re-evaluated.

Step 4: Putting It All Together

When an expression like (+ 1 (* 2 3)) is processed:

  1. Pass 1: The nesting rule matches. The % modifier forces the inner part (* 2 3) to be evaluated first. The base case rule transforms (* 2 3) into 6.
  2. The expression becomes (+ 1 6).
  3. Rewind: The transformer re-scans the new string (+ 1 6).
  4. Pass 2: The base case rule ({op:1} {a:1} {b:1}) matches (+ 1 6) and evaluates it to 7.

⚖️ Why uCalc? (Comparative Analysis)

Building a LISP interpreter from scratch is a classic computer science exercise that involves:

  1. Lexical Analysis: Writing a tokenizer to handle parentheses, symbols, and numbers.
  2. Syntactic Analysis: Building a parser to construct an Abstract Syntax Tree (AST) from the token stream.
  3. Evaluation: Writing an evaluator that walks the AST to compute the result.

uCalc allows us to skip almost all of this work. We leverage:

  • uCalc's built-in, high-performance tokenizer.
  • uCalc's powerful Transformer to handle the syntactic analysis (parsing) declaratively.
  • uCalc's mature evaluation engine to handle the final computation.

We are simply building a lightweight "syntactic frontend" that translates one syntax into another that the engine already understands. This dramatically reduces the complexity and amount of code required.

Examples

A simple LISP transpiler that handles only binary operators.
				
					using uCalcSoftware;

var uc = new uCalc();
var t = uc.ExpressionTransformer;
t.FromTo("({op:1} {a:1} {b:1})", "({a} {op} {b})");

Console.WriteLine("LISP: (+ 10 20)");
Console.WriteLine($"uCalc: {t.Transform("(+ 10 20)")}");
Console.WriteLine($"Result: {uc.Eval("(+ 10 20)")}");
				
			
LISP: (+ 10 20)
uCalc: (10 + 20)
Result: 30
				
					#include <iostream>
#include "uCalc.h"

using namespace std;
using namespace uCalcSoftware;

int main() {
   uCalc uc;
   auto t = uc.ExpressionTransformer();
   t.FromTo("({op:1} {a:1} {b:1})", "({a} {op} {b})");

   cout << "LISP: (+ 10 20)" << endl;
   cout << "uCalc: " << t.Transform("(+ 10 20)") << endl;
   cout << "Result: " << uc.Eval("(+ 10 20)") << endl;
}
				
			
LISP: (+ 10 20)
uCalc: (10 + 20)
Result: 30
				
					Imports System
Imports uCalcSoftware
Public Module Program
   Public Sub Main()
      Dim uc As New uCalc()
      Dim t = uc.ExpressionTransformer
      t.FromTo("({op:1} {a:1} {b:1})", "({a} {op} {b})")
      
      Console.WriteLine("LISP: (+ 10 20)")
      Console.WriteLine($"uCalc: {t.Transform("(+ 10 20)")}")
      Console.WriteLine($"Result: {uc.Eval("(+ 10 20)")}")
   End Sub
End Module
				
			
LISP: (+ 10 20)
uCalc: (10 + 20)
Result: 30
A practical LISP interpreter that handles variadic (multiple arguments) and nested expressions.
				
					using uCalcSoftware;

var uc = new uCalc();
var t = uc.NewTransformer();
t.DefaultRuleSet.RewindOnChange = true;

// {@@Eval} evaluates the expression obtained by concatinating the captured
// elements {op} for operator, and {a} and {b} for the two numbers.
// These elements are strings and do not need curly braces within {@@Eval}
// :1 tels it to capture one token at a time.
t.FromTo("({op:1} {a:1} {b:1})", "{@@Eval: a + op + b}");

// If there are more than to numbers, as indicated by {more}, then the first
// two numbers are evaluated and put back into the list.  The operator remains.
t.FromTo("({op:1} {a:1} {b:1} {more})", "({op} {@@Eval: a + op + b} {more})");

// If a nested expression is present, it is evaluated immediately, by using
// % in {expr%} and the section is reprocessed again with the resulting value
// since .@RewindOnChange(true) was set.
t.FromTo("({op:1} {expr%: ({exp})}", "({op} {expr}");
t.FromTo("({op:1} {a:1} {expr%: ({exp})}", "({op} {a} {expr}");

// --- Test Cases ---
Console.WriteLine("--- Simple Binary ---");
var expr = "(- 100 25)";
Console.WriteLine($"LISP: {expr}");
Console.WriteLine($"Result: {t.Transform(expr)}");
Console.WriteLine("");

Console.WriteLine("--- Variadic (Multiple Args) ---");
expr = "(* 2 3 4)";
Console.WriteLine($"LISP: {expr}");
Console.WriteLine($"Result: {t.Transform(expr)}");
Console.WriteLine("");

Console.WriteLine("--- Nested Expressions ---");
expr = "(/ 100 (+ 5 5))";
Console.WriteLine($"LISP: {expr}");
Console.WriteLine($"Result: {t.Transform(expr)}");
expr = "(+ (* (- 100 (/ 80 4)) 3) (- (* (+ 5 3) (- 12 7)) (+ (* 2 3) 4)))";
Console.WriteLine($"LISP: {expr}");
Console.WriteLine($"Result: {t.Transform(expr)}");
Console.WriteLine("");
				
			
--- Simple Binary ---
LISP: (- 100 25)
Result: 75

--- Variadic (Multiple Args) ---
LISP: (* 2 3 4)
Result: 24

--- Nested Expressions ---
LISP: (/ 100 (+ 5 5))
Result: 10
LISP: (+ (* (- 100 (/ 80 4)) 3) (- (* (+ 5 3) (- 12 7)) (+ (* 2 3) 4)))
Result: 270
				
					#include <iostream>
#include "uCalc.h"

using namespace std;
using namespace uCalcSoftware;

int main() {
   uCalc uc;
   auto t = uc.NewTransformer();
   t.DefaultRuleSet().RewindOnChange(true);

   // {@@Eval} evaluates the expression obtained by concatinating the captured
   // elements {op} for operator, and {a} and {b} for the two numbers.
   // These elements are strings and do not need curly braces within {@@Eval}
   // :1 tels it to capture one token at a time.
   t.FromTo("({op:1} {a:1} {b:1})", "{@@Eval: a + op + b}");

   // If there are more than to numbers, as indicated by {more}, then the first
   // two numbers are evaluated and put back into the list.  The operator remains.
   t.FromTo("({op:1} {a:1} {b:1} {more})", "({op} {@@Eval: a + op + b} {more})");

   // If a nested expression is present, it is evaluated immediately, by using
   // % in {expr%} and the section is reprocessed again with the resulting value
   // since .@RewindOnChange(true) was set.
   t.FromTo("({op:1} {expr%: ({exp})}", "({op} {expr}");
   t.FromTo("({op:1} {a:1} {expr%: ({exp})}", "({op} {a} {expr}");

   // --- Test Cases ---
   cout << "--- Simple Binary ---" << endl;
   auto expr = "(- 100 25)";
   cout << "LISP: " << expr << endl;
   cout << "Result: " << t.Transform(expr) << endl;
   cout << "" << endl;

   cout << "--- Variadic (Multiple Args) ---" << endl;
   expr = "(* 2 3 4)";
   cout << "LISP: " << expr << endl;
   cout << "Result: " << t.Transform(expr) << endl;
   cout << "" << endl;

   cout << "--- Nested Expressions ---" << endl;
   expr = "(/ 100 (+ 5 5))";
   cout << "LISP: " << expr << endl;
   cout << "Result: " << t.Transform(expr) << endl;
   expr = "(+ (* (- 100 (/ 80 4)) 3) (- (* (+ 5 3) (- 12 7)) (+ (* 2 3) 4)))";
   cout << "LISP: " << expr << endl;
   cout << "Result: " << t.Transform(expr) << endl;
   cout << "" << endl;
}
				
			
--- Simple Binary ---
LISP: (- 100 25)
Result: 75

--- Variadic (Multiple Args) ---
LISP: (* 2 3 4)
Result: 24

--- Nested Expressions ---
LISP: (/ 100 (+ 5 5))
Result: 10
LISP: (+ (* (- 100 (/ 80 4)) 3) (- (* (+ 5 3) (- 12 7)) (+ (* 2 3) 4)))
Result: 270
				
					Imports System
Imports uCalcSoftware
Public Module Program
   Public Sub Main()
      Dim uc As New uCalc()
      Dim t = uc.NewTransformer()
      t.DefaultRuleSet.RewindOnChange = true
      
      '// {@@Eval} evaluates the expression obtained by concatinating the captured
      '// elements {op} for operator, and {a} and {b} for the two numbers.
      '// These elements are strings and do not need curly braces within {@@Eval}
      '// :1 tels it to capture one token at a time.
      t.FromTo("({op:1} {a:1} {b:1})", "{@@Eval: a + op + b}")
      
      '// If there are more than to numbers, as indicated by {more}, then the first
      '// two numbers are evaluated and put back into the list.  The operator remains.
      t.FromTo("({op:1} {a:1} {b:1} {more})", "({op} {@@Eval: a + op + b} {more})")
      
      '// If a nested expression is present, it is evaluated immediately, by using
      '// % in {expr%} and the section is reprocessed again with the resulting value
      '// since .@RewindOnChange(true) was set.
      t.FromTo("({op:1} {expr%: ({exp})}", "({op} {expr}")
      t.FromTo("({op:1} {a:1} {expr%: ({exp})}", "({op} {a} {expr}")
      
      '// --- Test Cases ---
      Console.WriteLine("--- Simple Binary ---")
      Dim expr = "(- 100 25)"
      Console.WriteLine($"LISP: {expr}")
      Console.WriteLine($"Result: {t.Transform(expr)}")
      Console.WriteLine("")
      
      Console.WriteLine("--- Variadic (Multiple Args) ---")
      expr = "(* 2 3 4)"
      Console.WriteLine($"LISP: {expr}")
      Console.WriteLine($"Result: {t.Transform(expr)}")
      Console.WriteLine("")
      
      Console.WriteLine("--- Nested Expressions ---")
      expr = "(/ 100 (+ 5 5))"
      Console.WriteLine($"LISP: {expr}")
      Console.WriteLine($"Result: {t.Transform(expr)}")
      expr = "(+ (* (- 100 (/ 80 4)) 3) (- (* (+ 5 3) (- 12 7)) (+ (* 2 3) 4)))"
      Console.WriteLine($"LISP: {expr}")
      Console.WriteLine($"Result: {t.Transform(expr)}")
      Console.WriteLine("")
   End Sub
End Module
				
			
--- Simple Binary ---
LISP: (- 100 25)
Result: 75

--- Variadic (Multiple Args) ---
LISP: (* 2 3 4)
Result: 24

--- Nested Expressions ---
LISP: (/ 100 (+ 5 5))
Result: 10
LISP: (+ (* (- 100 (/ 80 4)) 3) (- (* (+ 5 3) (- 12 7)) (+ (* 2 3) 4)))
Result: 270
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