Refactor AST-Tool evaluation logic

Consolidate AST source code evaluation logic to handle script strings,
file paths, and stdin more uniformly. Introduce a `read_stdin` helper
function.

src/bin/ast.rs

@@ -2,6 +2,7 @@ use clap::Parser;
 use myc::ast::environment::Environment;
 use myc::utils::tester;
 use std::fs;
+use std::io::{self, IsTerminal, Read};
 use std::path::PathBuf;
 
 #[derive(Parser)]
@@ -73,20 +74,28 @@ fn main() {
         return;
     }
 
-    if let Some(script_str) = cli.eval {
-        if cli.dump {
-            match env.dump_ast(&script_str) {
-                Ok(dump) => println!("{}", dump),
-                Err(e) => eprintln!("Error dumping AST: {}", e),
-            }
-        } else if cli.trace {
-            execute_trace(&env, &script_str);
-        } else {
-            execute(&env, &script_str);
-        }
+    // Determine the source code to execute
+    let source = if let Some(script_str) = cli.eval {
+        Some(script_str)
     } else if let Some(file_path) = cli.file {
+        if file_path.to_str() == Some("-") {
+            read_stdin()
+        } else {
             match fs::read_to_string(&file_path) {
-            Ok(content) => {
+                Ok(content) => Some(content),
+                Err(e) => {
+                    eprintln!("Error reading file {:?}: {}", file_path, e);
+                    std::process::exit(1);
+                }
+            }
+        }
+    } else if !io::stdin().is_terminal() {
+        read_stdin()
+    } else {
+        None
+    };
+
+    if let Some(content) = source {
         if cli.dump {
             match env.dump_ast(&content) {
                 Ok(dump) => println!("{}", dump),
@@ -97,17 +106,22 @@ fn main() {
         } else {
             execute(&env, &content);
         }
-            }
-            Err(e) => {
-                eprintln!("Error reading file {:?}: {}", file_path, e);
-                std::process::exit(1);
-            }
-        }
     } else {
         println!("MYC AST Compiler CLI. Use --help for usage.");
     }
 }
 
+fn read_stdin() -> Option<String> {
+    let mut buffer = String::new();
+    match io::stdin().read_to_string(&mut buffer) {
+        Ok(_) => Some(buffer),
+        Err(e) => {
+            eprintln!("Error reading from stdin: {}", e);
+            std::process::exit(1);
+        }
+    }
+}
+
 fn execute(env: &Environment, source: &str) {
     let result = env.compile(source);
     for diag in &result.diagnostics.items {