Change the structure of the .erg directory

compiler/erg_common/env.rs

@@ -0,0 +1,15 @@
+use std::path::PathBuf;
+
+pub fn erg_path() -> Option<PathBuf> {
+    option_env!("ERG_PATH")
+        .or_else(|| option_env!("CARGO_ERG_PATH"))
+        .map(PathBuf::from)
+}
+
+pub fn erg_std_path() -> Option<PathBuf> {
+    erg_path().map(|path| path.join("lib").join("std"))
+}
+
+pub fn erg_external_lib_path() -> Option<PathBuf> {
+    erg_path().map(|path| path.join("lib").join("external"))
+}

compiler/erg_common/lib.rs

@@ -7,6 +7,7 @@ pub mod color;
 pub mod config;
 pub mod datetime;
 pub mod dict;
+pub mod env;
 pub mod error;
 pub mod fxhash;
 pub mod help_messages;

compiler/erg_compiler/build.rs

@@ -14,20 +14,36 @@ fn main() -> std::io::Result<()> {
         + "/.erg";
     if !path::Path::new(&erg_path).exists() {
         fs::create_dir(&erg_path)?;
-        fs::create_dir(format!("{erg_path}/std"))?;
     }
     println!("cargo:rustc-env=CARGO_ERG_PATH={erg_path}");
-    // println!("cargo:rustc-env=CARGO_ERG_STD_PATH={erg_path}/std");
     // create a std library in ".erg"
-    for res in fs::read_dir("std")? {
+    copy_dir(&erg_path, "lib")?;
+    Ok(())
+}
+
+fn copy_dir(erg_path: &str, path: &str) -> std::io::Result<()> {
+    let full_path = format!("{erg_path}/{path}");
+    if !path::Path::new(&full_path).exists() {
+        fs::create_dir(&full_path)?;
+    }
+    let mut dirs = vec![];
+    for res in fs::read_dir(path)? {
         let entry = res?;
-        let path = entry.path();
-        let filename = path
-            .file_name()
-            .expect("this is not a file")
-            .to_str()
-            .unwrap();
-        fs::copy(&path, format!("{erg_path}/std/{filename}"))?;
+        let entry_path = entry.path();
+        if entry_path.is_dir() {
+            dirs.push(entry);
+        } else {
+            let filename = entry_path
+                .file_name()
+                .expect("this is not a file")
+                .to_str()
+                .unwrap();
+            let filename = format!("{full_path}/{filename}");
+            fs::copy(&entry_path, filename)?;
+        }
+    }
+    for dir in dirs {
+        copy_dir(erg_path, dir.path().to_str().unwrap())?;
     }
     Ok(())
 }

compiler/erg_compiler/codegen.rs

@@ -8,6 +8,7 @@ use crate::ty::codeobj::{CodeObj, CodeObjFlags};
 use erg_common::astr::AtomicStr;
 use erg_common::cache::CacheSet;
 use erg_common::config::{ErgConfig, Input};
+use erg_common::env::erg_std_path;
 use erg_common::error::{ErrorDisplay, Location};
 use erg_common::opcode::Opcode;
 use erg_common::traits::{Locational, Stream};
@@ -2052,8 +2053,7 @@ impl CodeGenerator {
     }
 
     fn load_prelude_py(&mut self) {
-        if let Some(erg_path) = option_env!("ERG_PATH").or_else(|| option_env!("CARGO_ERG_PATH")) {
-            let std_path = std::path::Path::new(erg_path).join("std");
+        if let Some(std_path) = erg_std_path() {
             self.emit_global_import_items(
                 Identifier::public("sys"),
                 vec![(