feat: only allow identifiers as string interpolation placeholders

lib/pinc_backend/Pinc_Interpreter.ml

@@ -567,8 +567,11 @@ and eval_string_template ~state template =
 
                 match string_template.Ast.string_template_desc with
                 | StringText s -> s
-                | StringInterpolation e ->
-                    eval_expression ~state e |> State.get_output |> Value.to_string)
+                | StringInterpolation (Lowercase_Id (id, loc)) ->
+                    id
+                    |> eval_lowercase_identifier ~loc ~state
+                    |> State.get_output
+                    |> Value.to_string)
          |> String.concat ""
          |> Value.of_string
               ~value_loc:(Location.make ~s:!start_loc.loc_start ~e:!end_loc.loc_end ()))

lib/pinc_frontend/Pinc_Ast.ml

@@ -54,7 +54,7 @@ and string_template = {
 }
 
 and string_template_desc =
-  | StringInterpolation of expression
+  | StringInterpolation of lowercase_identifier
   | StringText of string
 
 and expression = {

lib/pinc_frontend/Pinc_Parser.ml

@@ -64,7 +64,7 @@ let make ~filename src =
 ;;
 
 module Helpers = struct
-  let expect_identifier ?(typ = `All) t =
+  let expect_identifier ?error_message ?(typ = `All) t =
     let location =
       Location.make ~s:t.token.location.loc_start ~e:t.token.location.loc_end ()
     in
@@ -78,36 +78,37 @@ module Helpers = struct
     | token when typ = `Lower ->
         Diagnostics.error
           location
-          (match token with
-          | Token.IDENT_UPPER i ->
+          (match (error_message, token) with
+          | Some message, _ -> message
+          | None, Token.IDENT_UPPER i ->
               "Expected to see a lowercase identifier at this point. Did you mean "
               ^ String.lowercase_ascii i
               ^ " instead of "
               ^ i
               ^ "?"
-          | t when Token.is_keyword t ->
-              "`" ^ Token.to_string t ^ "` is a keyword. Please choose another name."
-          | t ->
+          | None, t ->
               "Expected to see a lowercase identifier at this point. Instead saw "
               ^ Token.to_string t)
     | token when typ = `Upper ->
         Diagnostics.error
           location
-          (match token with
-          | Token.IDENT_LOWER i ->
+          (match (error_message, token) with
+          | Some message, _ -> message
+          | None, Token.IDENT_LOWER i ->
               "Expected to see an uppercase identifier at this point. Did you mean "
               ^ String.capitalize_ascii i
               ^ " instead of "
               ^ i
               ^ "?"
-          | _ -> "Expected to see an uppercase identifier at this point.")
+          | None, _ -> "Expected to see an uppercase identifier at this point.")
     | token ->
         Diagnostics.error
           location
-          (match token with
-          | t when Token.is_keyword t ->
+          (match (error_message, token) with
+          | Some message, _ -> message
+          | None, t when Token.is_keyword t ->
               "\"" ^ Token.to_string t ^ "\" is a keyword. Please choose another name."
-          | _ -> "Expected to see an identifier at this point.")
+          | None, _ -> "Expected to see an identifier at this point.")
   ;;
 
   let separated_list ~sep ~fn t =
@@ -146,9 +147,18 @@ module Rules = struct
           Some (Ast.StringText s)
       | Token.OPEN_TEMPLATE_LITERAL ->
           next t;
-          let* expression = parse_expression t in
+          let identifier =
+            t
+            |> Helpers.expect_identifier
+                 ~typ:`Lower
+                 ~error_message:
+                   "Only lowercase identifiers are allowed as string interpolation \
+                    placeholders.\n\
+                    If you want to use more complex constructs, you can always assign \
+                    them to a let binding."
+          in
           t |> expect Token.RIGHT_PAREN;
-          Some (Ast.StringInterpolation expression)
+          Some (Ast.StringInterpolation (Lowercase_Id identifier))
       | _ -> None
     in
     let string_template_end = t.token.location.loc_end in

test/context.t/SubComponent.pi

@@ -1,7 +1,8 @@
 component SubComponent {
   let background = #GetContext;
 
-  <div class="Sub $(if (background == "grey") "Sub--grey" else "")">
+  let backgroundClass = if (background == "grey") "Sub--grey" else "";
+  <div class="Sub $(backgroundClass)">
     <p>Faucibus mus velit ut iaculis cubilia taciti conubia, ad magna sem mattis tincidunt eleifend, egestas eget neque curae molestie donec. Magna ipsum netus euismod vitae justo dictum adipiscing lobortis, posuere tortor penatibus tempus laoreet habitant orci, turpis blandit nullam ornare fames nostra sed.</p>
   </div>
 }

test/parse_error.t/run.t

@@ -34,6 +34,7 @@
 
   Expected expression as right hand side of let declaration
   [1]
+
   $ print ./mutation_without_expression.pi Component
 
   ERROR in file ./mutation_without_expression.pi:3:3-12
@@ -44,3 +45,15 @@
 
   Expected expression as right hand side of mutation statement
   [1]
+
+  $ print ./string_interpolation.pi Component
+
+  ERROR in file ./string_interpolation.pi:4:25-27
+
+     3 │ 
+     4 │   <div class="Section $(if fail "A" else "B")">
+     5 │     {fail}
+
+  Only lowercase identifiers are allowed as string interpolation placeholders.
+  If you want to use more complex constructs, you can always assign them to a let binding.
+  [1]

test/parse_error.t/string_interpolation.pi

@@ -0,0 +1,7 @@
+component Component {
+  let fail = #String
+
+  <div class="Section $(if fail "A" else "B")">
+    {fail}
+  </div>
+}

test/template.t/data.pi

@@ -3,10 +3,9 @@ component Component(
   icon: "/images/icons/group.svg",
   group: "Structure",
 ) {
-
-  let name = "String Template";
   let intro = true;
-  let content = "Hello, $(if (intro) { name } else { "World" })!";
+  let name = if (intro) "String Template" else "World";
+  let content = "Hello, $(name)!";
 
   <section class="Section">
     <div />