handle echo not followed by an expression to print

compiler-core/src/error.rs

@@ -3079,6 +3079,22 @@ Try: _{}", kind_str.to_title_case(), name.to_snake_case()),
                         }),
                     }
                 },
+
+                TypeError::EchoWithNoFollowingExpression { location } => Diagnostic {
+                    title: format!("Invalid echo use"),
+                    text: wrap("The `echo` keyword should be followed by a value to print."),
+                    hint: None,
+                    level: Level::Error,
+                    location: Some(Location {
+                        label: Label {
+                            text: Some("I was expecting a value after this".into()),
+                            span: *location,
+                        },
+                        path: path.clone(),
+                        src: src.clone(),
+                        extra_labels: vec![],
+                    }),
+                },
             }
         }).collect_vec(),
 

compiler-core/src/type_/error.rs

@@ -550,6 +550,20 @@ pub enum Error {
         kind: Named,
         name: EcoString,
     },
+
+    /// When the echo keyword is not followed by an expression to be printed.
+    /// The only place where echo is allowed to appear on its own is as a step
+    /// of a pipeline, otherwise omitting the expression will result in this
+    /// error. For example:
+    ///
+    /// ```gleam
+    /// call(echo, 1, 2)
+    /// //   ^^^^ Error!
+    /// ```
+    ///
+    EchoWithNoFollowingExpression {
+        location: SrcSpan,
+    },
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -938,6 +952,7 @@ impl Error {
             }
             | Error::UseFnDoesntTakeCallback { location, .. }
             | Error::UseFnIncorrectArity { location, .. }
+            | Error::EchoWithNoFollowingExpression { location }
             | Error::BadName { location, .. } => location.start,
             Error::UnknownLabels { unknown, .. } => {
                 unknown.iter().map(|(_, s)| s.start).min().unwrap_or(0)

compiler-core/src/type_/expression.rs

@@ -479,12 +479,7 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
             let typed_expression = self.infer(*expression);
             Ok(todo!("typed expr ECHO"))
         } else {
-            todo!(
-                "
-echo with no expression after it
-should only be in a pipeline
-"
-            )
+            Err(Error::EchoWithNoFollowingExpression { location })
         }
     }
 

compiler-core/src/type_/pipe.rs

@@ -123,6 +123,13 @@ impl<'a, 'b, 'c> PipeTyper<'a, 'b, 'c> {
                     }
                 }
 
+                UntypedExpr::Echo {
+                    location,
+                    expression: None,
+                } => {
+                    todo!("This echo is actually ok!")
+                }
+
                 // right(left)
                 call => self.infer_apply_pipe(call)?,
             };

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__echo_followed_by_no_expression.snap

@@ -0,0 +1,11 @@
+---
+source: compiler-core/src/type_/tests/errors.rs
+expression: echo
+---
+error: Invalid echo use
+  ┌─ /src/one/two.gleam:1:1
+  │
+1 │ echo
+  │ ^^^^ I was expecting a value after this
+
+The `echo` keyword should be followed by a value to print.

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__echo_followed_by_no_expression_10.snap

@@ -0,0 +1,11 @@
+---
+source: compiler-core/src/type_/tests/errors.rs
+expression: "\npub fn main() {\n  echo\n  |> todo\n}\n"
+---
+error: Invalid echo use
+  ┌─ /src/one/two.gleam:3:3
+  │
+3 │   echo
+  │   ^^^^ I was expecting a value after this
+
+The `echo` keyword should be followed by a value to print.

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__echo_followed_by_no_expression_2.snap

@@ -0,0 +1,11 @@
+---
+source: compiler-core/src/type_/tests/errors.rs
+expression: "\n  pub fn wibble(a) { a }\n\n  pub fn main() {\n    wibble(echo)\n  }\n"
+---
+error: Invalid echo use
+  ┌─ /src/one/two.gleam:5:12
+  │
+5 │     wibble(echo)
+  │            ^^^^ I was expecting a value after this
+
+The `echo` keyword should be followed by a value to print.

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__echo_followed_by_no_expression_3.snap

@@ -0,0 +1,11 @@
+---
+source: compiler-core/src/type_/tests/errors.rs
+expression: "\n  pub fn main() {\n    echo + 1\n  }\n"
+---
+error: Invalid echo use
+  ┌─ /src/one/two.gleam:3:5
+  │
+3 │     echo + 1
+  │     ^^^^ I was expecting a value after this
+
+The `echo` keyword should be followed by a value to print.

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__echo_followed_by_no_expression_4.snap

@@ -0,0 +1,11 @@
+---
+source: compiler-core/src/type_/tests/errors.rs
+expression: "\n  pub fn main() {\n    \"wibble\" <> echo\n  }\n"
+---
+error: Invalid echo use
+  ┌─ /src/one/two.gleam:3:17
+  │
+3 │     "wibble" <> echo
+  │                 ^^^^ I was expecting a value after this
+
+The `echo` keyword should be followed by a value to print.

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__echo_followed_by_no_expression_5.snap

@@ -0,0 +1,11 @@
+---
+source: compiler-core/src/type_/tests/errors.rs
+expression: "\npub fn main() {\n  panic as echo\n}\n"
+---
+error: Invalid echo use
+  ┌─ /src/one/two.gleam:3:12
+  │
+3 │   panic as echo
+  │            ^^^^ I was expecting a value after this
+
+The `echo` keyword should be followed by a value to print.

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__echo_followed_by_no_expression_6.snap

@@ -0,0 +1,11 @@
+---
+source: compiler-core/src/type_/tests/errors.rs
+expression: "\npub fn main() {\n  [echo, 1, 2]\n}\n"
+---
+error: Invalid echo use
+  ┌─ /src/one/two.gleam:3:4
+  │
+3 │   [echo, 1, 2]
+  │    ^^^^ I was expecting a value after this
+
+The `echo` keyword should be followed by a value to print.

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__echo_followed_by_no_expression_7.snap

@@ -0,0 +1,11 @@
+---
+source: compiler-core/src/type_/tests/errors.rs
+expression: "\npub fn main() {\n  #(1, echo)\n}\n"
+---
+error: Invalid echo use
+  ┌─ /src/one/two.gleam:3:8
+  │
+3 │   #(1, echo)
+  │        ^^^^ I was expecting a value after this
+
+The `echo` keyword should be followed by a value to print.

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__echo_followed_by_no_expression_8.snap

@@ -0,0 +1,11 @@
+---
+source: compiler-core/src/type_/tests/errors.rs
+expression: "\npub fn main() {\n  todo\n  |> fn(_) { echo }\n  |> todo\n}\n"
+---
+error: Invalid echo use
+  ┌─ /src/one/two.gleam:4:14
+  │
+4 │   |> fn(_) { echo }
+  │              ^^^^ I was expecting a value after this
+
+The `echo` keyword should be followed by a value to print.

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__echo_followed_by_no_expression_9.snap

@@ -0,0 +1,11 @@
+---
+source: compiler-core/src/type_/tests/errors.rs
+expression: "\npub fn main() {\n  todo\n  |> { echo }\n  |> todo\n}\n"
+---
+error: Invalid echo use
+  ┌─ /src/one/two.gleam:4:8
+  │
+4 │   |> { echo }
+  │        ^^^^ I was expecting a value after this
+
+The `echo` keyword should be followed by a value to print.