Feature/error pythonic import

changelog/v1.9.md

@@ -80,6 +80,10 @@
   size.
   ([Surya Rose](https://github.com/GearsDatapacks))
 
+- Gleam will now issue a help message when attempting to import using Python's
+  import syntax.
+  ([Zij-IT](https://github.com/zij-it))
+
 ### Build tool
 
 - The build tool now supports Git dependencies. For example:

compiler-core/src/parse.rs

@@ -2686,8 +2686,32 @@ where
         let mut unqualified_values = vec![];
         let mut unqualified_types = vec![];
 
-        if self.maybe_one(&Token::Dot).is_some() {
-            let _ = self.expect_one(&Token::LeftBrace)?;
+        if let Some((dot_start, dot_end)) = self.maybe_one(&Token::Dot) {
+            let _ = self.expect_one(&Token::LeftBrace).map_err(|e| {
+                // If the module does contain a '/', then it's unlikely that the user
+                // intended for the import to be pythonic, so skip this.
+                if module.contains('/') {
+                    return e;
+                }
+
+                // Catch `import gleam.io` and provide a more helpful error...
+                let ParseErrorType::UnexpectedToken {
+                    token: Token::Name { name } | Token::UpName { name },
+                    ..
+                } = &e.error
+                else {
+                    return e;
+                };
+
+                ParseError {
+                    error: ParseErrorType::PythonicImport {
+                        module: module.as_str().into(),
+                        item: name.clone(),
+                    },
+                    location: SrcSpan::new(dot_start, dot_end),
+                }
+            })?;
+
             let parsed = self.parse_unqualified_imports()?;
             unqualified_types = parsed.types;
             unqualified_values = parsed.values;

compiler-core/src/parse/error.rs

@@ -338,6 +338,16 @@ utf16_codepoint, utf32_codepoint, signed, unsigned, big, little, native, size, u
                 "This attribute cannot be used on a variant.",
                 vec!["Hint: Did you mean `@deprecated`?".into()],
             ),
+            ParseErrorType::PythonicImport { module, item } => (
+                "I was expecting either `/` or `.{` here.",
+                vec![
+                    "This syntax for an import is incorrect. Perhaps you meant:".into(),
+                    format!(" - `import {module}/{item}` to import the `{item}` \
+module from the `{module}` namespace"),
+                    format!(" - `import {module}.{{{item}}}` to import the \
+`{item}` value from the `{module}` module"),
+                ]
+            )
         }
     }
 }
@@ -411,6 +421,11 @@ pub enum ParseErrorType {
     TypeConstructorNoArguments,           // let a : Int()
     TypeDefinitionNoArguments,            // pub type Wibble() { ... }
     UnknownAttributeRecordVariant, // an attribute was used that is not know for a custom type variant
+    // a Python-like import was written, such as `import gleam.io`, instead of `import gleam/io`
+    PythonicImport {
+        module: EcoString,
+        item: EcoString,
+    },
 }
 
 impl LexicalError {

compiler-core/src/parse/snapshots/gleam_core__parse__tests__doesnt_issue_special_error_for_pythonic_import_if_slash.snap

@@ -0,0 +1,16 @@
+---
+source: compiler-core/src/parse/tests.rs
+expression: import one/two.three
+---
+----- SOURCE CODE
+import one/two.three
+
+----- ERROR
+error: Syntax error
+  ┌─ /src/parse/error.gleam:1:16
+  │
+1 │ import one/two.three
+  │                ^^^^^ I was not expecting this
+
+Found a name, expected one of: 
+- `{`

compiler-core/src/parse/snapshots/gleam_core__parse__tests__special_error_for_pythonic_import.snap

@@ -0,0 +1,17 @@
+---
+source: compiler-core/src/parse/tests.rs
+expression: import gleam.io
+---
+----- SOURCE CODE
+import gleam.io
+
+----- ERROR
+error: Syntax error
+  ┌─ /src/parse/error.gleam:1:13
+  │
+1 │ import gleam.io
+  │             ^ I was expecting either `/` or `.{` here.
+
+This syntax for an import is incorrect. Perhaps you meant:
+ - `import gleam/io` to import the `io` module from the `gleam` namespace
+ - `import gleam.{io}` to import the `io` module from the `gleam` module

compiler-core/src/parse/tests.rs

@@ -1766,3 +1766,13 @@ fn nested_tuple_access_after_function() {
 fn case_expression_without_body() {
     assert_parse!("case a");
 }
+
+#[test]
+fn special_error_for_pythonic_import() {
+    assert_module_error!("import gleam.io");
+}
+
+#[test]
+fn doesnt_issue_special_error_for_pythonic_import_if_slash() {
+    assert_module_error!("import one/two.three");
+}