better function names and improve documentation

Author: scristobal

compiler-core/src/type_/environment.rs

@@ -596,8 +596,14 @@ impl Environment<'_> {
                                 // Check this in the hydrator, i.e. is it a created type
                                 let v = self.new_unbound_var();
                                 let _ = ids.insert(*id, v.clone());
-                                let new_id = self.previous_uid();
-                                self.names.map_new_variable(*id, new_id);
+
+                                // Preserve any user-provided name from the original generic variable
+                                // for the new unbound variable. This ensures error messages and type
+                                // displays continue to use meaningful names (e.g., "something") rather
+                                // than auto-generated ones (e.g., "a", "b").
+                                let v_id = self.previous_uid();
+                                self.names.reassign_type_variable_alias(*id, v_id);
+
                                 return v;
                             }
                         }

compiler-core/src/type_/printer.rs

@@ -181,9 +181,11 @@ impl Names {
         _ = self.type_variables.insert(id, local_alias.clone());
     }
 
-    pub fn map_new_variable(&mut self, old_id: u64, new_id: u64) {
-        if let Some(alias) = self.type_variables.get(&old_id) {
-            _ = self.type_variables.insert(new_id, alias.clone());
+    /// Assigns `source_id` alias to `target_id`. This is useful when type variables are unified
+    /// and we need to preserve the original name for consistent error messages and type display.
+    pub fn reassign_type_variable_alias(&mut self, source_id: u64, target_id: u64) {
+        if let Some(alias) = self.type_variables.get(&source_id) {
+            _ = self.type_variables.insert(target_id, alias.clone());
         }
     }