Trying a more object oriented approach and created an 'Object' struct to have metadata for the values the interpreter works with.

interpreter.c

@@ -1,9 +1,9 @@
 #include "interpreter.h"
 #include "token.h"
 
-void* Evaluate(Expr*);
+Object* Evaluate(Expr*);
-int IsTruthy(void*);
+int IsTruthy(Object*);
-int IsEqual(void*, void*);
+int IsEqual(Object*, Object*);
 
 const void* VisitLiteralExpression(Expr* expression) {
     return expression->expression.Literal->literal;
@@ -66,22 +66,31 @@ void* VisitUnaryExpression(Expr* expression) {
     return NULL; //Should be unreachable.
 }
 
-void* Evaluate(Expr* expression) {
+Object* Evaluate(Expr* expression) {
     //accept
     return NULL;
 }
 
-int IsTruthy(void* object) {
+int IsTruthy(Object* object) {
-    if (!object) return 0;
+    if (!object || object->instance == NIL) return 0;
-    if (*((int *) object) == 0) return 0;
+    if (object->instance == TRUE) return 0;
 
     return 1;
 }
 
-int IsEqual(void* a, void* b) {
+int IsEqual(Object* a, Object* b) {
-    if (!a && !b) return 1;
+    if (a->instance == NIL && b->instance == NIL) return 1;
-    if (!a) return 0;
+    if (a->instance == NIL) return 0;
+
+    if (b->instance == Number && b->instance == Number) {
+        return a->value.number == b->value.number;
+    }
+
+    if (a->instance == String && b->instance == String) {
+        int result = strcmp(a->value.string, b->value.string);
+
+        return result == 0;
+    }
     
-    //Oh this'll be fun...
     return 0;
 }

interpreter.h

@@ -4,7 +4,17 @@
 #include "expr.h"
 #include "token.h"
 
+typedef struct Object {
+    TokenType instance;
+    union {
+        char* string;
+        double number;
+        int boolean;
+    } value;
+} Object;
+
 const void* VisitLiteralExpression(Expr*);
 void* VisitGroupingExpression(Expr*);
+void* VisitBinaryExpression(Expr*);
 
 #endif

token.c

@@ -1,4 +1,6 @@
 #include "token.h"
+#include <stdlib.h>
+#include <string.h>
 
 KeyValuePair TokenTypeMappings[TOKENTYPE_MAPPINGS_COUNT] = {
     { "(", LParen}, { ")", RParen}, { ",", Comma }, { ".", Dot },
@@ -36,7 +38,7 @@ Token* CreateToken(const char* lexeme, void* literal, int line, TokenType type)
             return NULL;
         }
 
-        token->literal = mapping_result;
+        if (type != NIL) token->literal = mapping_result;
         token->lexeme = mapping_result;
     }
 
@@ -61,3 +63,31 @@ void FreeToken(Token* token) {
 
     free(token);
 }
+
+char* GetTokenStringValue(Token* token, int* length) {
+    length = 0;
+
+    if (!token) return NULL;
+    if (token->type != String) return NULL;
+
+    *length = strlen(token->lexeme);
+
+    if (length == 0) return NULL;
+
+    char* value = calloc(strlen(token->lexeme) + 1, sizeof(char));
+
+    strncpy(value, token->lexeme, *length);
+
+    return value;
+}
+
+int GetTokenNumberValue(Token* token, double* value) {
+    value = 0;
+
+    if (!token) return 0;
+    if (token->type != Number) return 0;
+
+    *value = *((double*)token->literal);
+
+    return 1;
+}

token.h

@@ -50,6 +50,8 @@ extern KeyValuePair TokenTypeMappings[TOKENTYPE_MAPPINGS_COUNT];
 
 //If the first parameter is NULL, the token creation will attempt to infer the lexeme from the TokenType.
 Token* CreateToken(const char*, void*, int, TokenType);
+char* GetTokenStringValue(Token*, int*);
+int GetTokenNumberValue(Token*, double*);
 void FreeToken(Token*);
 
 #endif