Fix exponential syntax (#7174)

* Check for digits after exponent char

* Add tests

* Update CHANGELOG

* Better variable name

* Update CHANGELOG

CHANGELOG.md

@@ -11,6 +11,7 @@
 > - :nail_care: [Polish]
 
 # 12.0.0-alpha.6 (Unreleased)
+- Fix exponential notation syntax. https://github.com/rescript-lang/rescript/pull/7174
 
 # 12.0.0-alpha.5
 

compiler/syntax/src/res_scanner.ml

@@ -209,24 +209,30 @@ let scan_identifier scanner =
 
 let scan_digits scanner ~base =
   if base <= 10 then
-    let rec loop scanner =
+    let rec loop scanner found_digits =
       match scanner.ch with
-      | '0' .. '9' | '_' ->
+      | '0' .. '9' ->
         next scanner;
-        loop scanner
-      | _ -> ()
+        loop scanner true
+      | '_' ->
+        next scanner;
+        loop scanner false
+      | _ -> found_digits
     in
-    loop scanner
+    loop scanner false
   else
-    let rec loop scanner =
+    let rec loop scanner found_digits =
       match scanner.ch with
       (* hex *)
-      | '0' .. '9' | 'a' .. 'f' | 'A' .. 'F' | '_' ->
+      | '0' .. '9' | 'a' .. 'f' | 'A' .. 'F' ->
+        next scanner;
+        loop scanner true
+      | '_' ->
         next scanner;
-        loop scanner
-      | _ -> ()
+        loop scanner false
+      | _ -> found_digits
     in
-    loop scanner
+    loop scanner false
 
 (* float: (0…9) { 0…9∣ _ } [. { 0…9∣ _ }] [(e∣ E) [+∣ -] (0…9) { 0…9∣ _ }]   *)
 let scan_number scanner =
@@ -251,25 +257,30 @@ let scan_number scanner =
         8)
     | _ -> 10
   in
-  scan_digits scanner ~base;
+  ignore (scan_digits scanner ~base);
 
   (*  *)
   let is_float =
     if '.' == scanner.ch then (
       next scanner;
-      scan_digits scanner ~base;
+      ignore (scan_digits scanner ~base);
       true)
     else false
   in
 
   (* exponent part *)
   let is_float =
+    let start_pos = position scanner in
     match scanner.ch with
     | 'e' | 'E' | 'p' | 'P' ->
       (match peek scanner with
       | '+' | '-' -> next2 scanner
       | _ -> next scanner);
-      scan_digits scanner ~base;
+      let end_pos = position scanner in
+      let found_digits = scan_digits scanner ~base in
+      if not found_digits then
+        scanner.err ~start_pos ~end_pos
+          (Diagnostics.message "Expected digits after exponential notation.");
       true
     | _ -> is_float
   in

tests/syntax_tests/data/parsing/errors/scanner/expected/exponent_notation.res.txt

@@ -0,0 +1,30 @@
+
+  Syntax error!
+  syntax_tests/data/parsing/errors/scanner/exponent_notation.res:7:10
+
+  5 │ let x = 1_e_1
+  6 │ 
+  7 │ let x = 1e
+  8 │ 
+  9 │ let x = 1_e_
+
+  Expected digits after exponential notation.
+
+
+  Syntax error!
+  syntax_tests/data/parsing/errors/scanner/exponent_notation.res:9:11
+
+   7 │ let x = 1e
+   8 │ 
+   9 │ let x = 1_e_
+  10 │ 
+  11 │ let x = 1.
+
+  Expected digits after exponential notation.
+
+let x = 1e1
+let x = 1e_1
+let x = 1_e_1
+let x = 1e
+let x = 1_e_
+let x = 1.

tests/syntax_tests/data/parsing/errors/scanner/exponent_notation.res

@@ -0,0 +1,11 @@
+let x = 1e1
+
+let x = 1e_1
+
+let x = 1_e_1
+
+let x = 1e
+
+let x = 1_e_
+
+let x = 1.