feat: add {Str, List}.from

crates/erg_compiler/context/initialize/classes.rs

@@ -1247,6 +1247,13 @@ impl Context {
             Visibility::BUILTIN_PUBLIC,
             Some(FUNC_ISPRINTABLE),
         );
+        str_.register_builtin_py_impl(
+            FUNC_FROM,
+            no_var_fn_met(Str, vec![kw(KW_NTH, Nat)], vec![], Str),
+            Immutable,
+            Visibility::BUILTIN_PUBLIC,
+            Some(FUNC_FROM_),
+        );
         let str_getitem_t = fn1_kw_met(Str, kw(KW_IDX, Nat | poly(RANGE, vec![ty_tp(Int)])), Str);
         str_.register_builtin_erg_impl(
             FUNDAMENTAL_GETITEM,
@@ -1796,6 +1803,20 @@ impl Context {
             Some(list_remove_all_t),
             list_remove_all,
         );
+        let list_from = no_var_fn_met(
+            unknown_len_list_t(T.clone()),
+            vec![kw(KW_NTH, Nat)],
+            vec![],
+            unknown_len_list_t(T.clone()),
+        )
+        .quantify();
+        list_.register_builtin_py_impl(
+            FUNC_FROM,
+            list_from,
+            Immutable,
+            Visibility::BUILTIN_PUBLIC,
+            Some(FUNC_FROM_),
+        );
         list_
             .register_trait(self, poly(INDEXABLE, vec![ty_tp(input), ty_tp(T.clone())]))
             .unwrap();

crates/erg_compiler/context/initialize/mod.rs

@@ -332,6 +332,8 @@ const FUNC_REMOVE: &str = "remove";
 const PROC_REMOVE: &str = "remove!";
 const FUNC_REMOVE_AT: &str = "remove_at";
 const FUNC_REMOVE_ALL: &str = "remove_all";
+const FUNC_FROM: &str = "from";
+const FUNC_FROM_: &str = "from_";
 const FUNC_POP: &str = "pop";
 const PROC_POP: &str = "pop!";
 const FUNC_CLEAR: &str = "clear";
@@ -681,6 +683,7 @@ const KW_FILLCHAR: &str = "fillchar";
 const KW_WIDTH: &str = "width";
 const KW_ARGS: &str = "args";
 const KW_KWARGS: &str = "kwargs";
+const KW_NTH: &str = "nth";
 const KW_IDX: &str = "idx";
 const KW_LHS: &str = "lhs";
 const KW_RHS: &str = "rhs";

crates/erg_compiler/context/inquire.rs

@@ -1349,7 +1349,9 @@ impl Context {
         }
         match self.get_attr_type_by_name(obj, attr_name, namespace) {
             Triple::Ok(method) => {
-                let def_t = self.instantiate_def_type(&method.definition_type).unwrap();
+                let def_t = self
+                    .instantiate_def_type(&method.definition_type)
+                    .map_err(|mut errs| errs.remove(0))?;
                 let list = UndoableLinkedList::new();
                 self.undoable_sub_unify(obj.ref_t(), &def_t, obj, &list, None)
                     // HACK: change this func's return type to TyCheckResult<Type>
@@ -1473,7 +1475,9 @@ impl Context {
         }
         match self.get_attr_type_by_name(obj, attr_name, namespace) {
             Triple::Ok(method) => {
-                let def_t = self.instantiate_def_type(&method.definition_type).unwrap();
+                let def_t = self
+                    .instantiate_def_type(&method.definition_type)
+                    .map_err(|mut errs| errs.remove(0))?;
                 let list = UndoableLinkedList::new();
                 self.undoable_sub_unify(obj.ref_t(), &def_t, obj, &list, None)
                     // HACK: change this func's return type to TyCheckResult<Type>

crates/erg_compiler/context/instantiate_spec.rs

@@ -2572,7 +2572,7 @@ impl Context {
                     };
                 }
                 let ty = new_set.iter().fold(Type::Never, |t, tp| {
-                    self.union(&t, &self.get_tp_t(tp).unwrap().derefine())
+                    self.union(&t, &self.get_tp_t(tp).unwrap_or(Obj).derefine())
                 });
                 if errs.is_empty() {
                     Ok(tp_enum(ty, new_set))

crates/erg_compiler/lib/core.d/List.d.er

@@ -56,3 +56,10 @@
     assert [1, 2, 3].reversed() == [3, 2, 1]
     '''
     reversed: |T: Type, N: Nat|(self: List(T, N)) -> List(T, N)
+    '''
+    Returns the array with the first `nth` elements removed.
+    '''
+    '''erg
+    assert [1, 2, 3, 4, 5].from(2) == [3, 4, 5]
+    '''
+    from: |T: Type|(self: List(T, _), nth: Nat) -> List(T, _)

crates/erg_compiler/lib/core.d/Str.d.er

@@ -181,3 +181,10 @@
     translate: (self: .Str, table: {Nat: Nat or NoneType}) -> .Str
     upper: (self: .Str) -> .Str
     zfill: (self: .Str, width: Nat) -> .Str
+    '''
+    Returns the substring of the string starting at the given index.
+    '''
+    '''erg
+    assert "abcde".from(2) == "cde"
+    '''
+    from: (self: Str, nth: Nat) -> Str

crates/erg_compiler/lib/core/_erg_list.py

@@ -137,6 +137,8 @@ class List(list):
             new.extend(deepcopy(self))
         return List(new)
 
+    def from_(self, nth: int):
+        return self[nth:]
 
 class UnsizedList:
     elem: object

crates/erg_compiler/lib/core/_erg_str.py

@@ -48,6 +48,8 @@ class Str(str):
         else:
             return str.__getitem__(self, index_or_slice)
 
+    def from_(self, nth: int):
+        return self[nth:]
 
 class StrMut(MutType):  # Inherits Str
     value: Str

crates/erg_compiler/lib/pystd/itertools.d.er

@@ -65,8 +65,8 @@
 .Islice(T) <: Output T
 .Islice(T) <: Iterable T
 .Islice(T).
-    __call__: (iterable: Iterable(T), start := Int, stop := Int, step := Int) -> .Islice(T)
-.islice: |T|(iterable: Iterable(T), start := Int, stop := Int, step := Int) -> .Islice(T)
+    __call__: (iterable: Iterable(T), start := Int or NoneType, stop := Int or NoneType, step := Int or NoneType) -> .Islice(T)
+.islice: |T|(iterable: Iterable(T), start := Int or NoneType, stop := Int or NoneType, step := Int or NoneType) -> .Islice(T)
 
 .Pairwise = 'pairwise': (T: Type) -> ClassType
 .Pairwise(T) <: Output T

tests/should_ok/list.er

@@ -38,3 +38,8 @@ l2 = [![1]].repeat 3
 l2[0].push! 2
 ans: List(List(Nat)) = [[1, 2], [1], [1]]
 assert l2 == ans
+
+l3 = [1, 2, 3, 4]
+assert l3.from(2) == [3, 4]
+assert l3.from(2).from(1) == [4]
+assert l3.from(5) == []