Fix #1272, leading underscore gets parsed funny

src/Parse/Helpers.hs

@@ -4,7 +4,7 @@ module Parse.Helpers
   ( module Parse.Primitives
   , SParser
   , qualifiedVar, qualifiedCapVar
-  , equals, rightArrow, hasType, comma, pipe, cons, dot, minus, underscore, lambda
+  , equals, rightArrow, hasType, comma, pipe, cons, dot, minus, lambda
   , leftParen, rightParen, leftSquare, rightSquare, leftCurly, rightCurly
   , addLocation, inContext
   , spaces, checkSpace, checkAligned, checkFreshLine
@@ -119,12 +119,6 @@ minus =
   symbol "-"
 
 
-{-# INLINE underscore #-}
-underscore :: Parser ()
-underscore =
-  symbol "_"
-
-
 {-# INLINE lambda #-}
 lambda :: Parser ()
 lambda =

src/Parse/Primitives.hs

@@ -5,7 +5,7 @@ module Parse.Primitives
   , run, runAt
   , try, deadend, hint, endOfFile
   , oneOf
-  , symbol, keyword, keywords
+  , symbol, underscore, keyword, keywords
   , lowVar, capVar, infixOp
   , getPosition, getCol
   , getContext, pushContext, popContext
@@ -318,6 +318,23 @@ moveCursor array offset length row col =
         moveCursor array (offset + 2) (length - 2) row (col + 1)
 
 
+underscore :: Parser ()
+underscore =
+  Parser $ \(State array offset length indent row col ctx) cok cerr _ eerr ->
+    if length == 0 || Text.unsafeIndex array offset /= 0x005F {- _ -} then
+      eerr noError
+
+    else if length >= 2 && isInnerVarChar (peekChar array (offset + 1)) then
+      cerr (ParseError row (col + 1) (BadUnderscore (peekChar array (offset + 1))))
+
+    else
+      let
+        !newState =
+          State array (offset + 1) (length - 1) indent row (col + 1) ctx
+      in
+        cok () newState noError
+
+
 
 -- KEYWORDS
 

src/Reporting/Error/Syntax.hs

@@ -75,6 +75,7 @@ data Problem
   | BadNumberHex
   | BadNumberZero
   | BadShader Text
+  | BadUnderscore Char
   | BadOp BadOp ContextStack
   | Theories ContextStack [Theory]
 
@@ -528,6 +529,15 @@ problemToReport subRegion problem =
         "I ran into a problem while parsing this GLSL block."
         (reflowParagraph msg)
 
+    BadUnderscore char ->
+      parseReport
+        ("An underscore cannot be followed by `" <> Text.singleton char <> "`.")
+        ( reflowParagraph $
+            "Variable names cannot start with an underscore in Elm. You can\
+            \ use an underscore as a \"wildcard\" that matches anything\
+            \ though. So maybe you forgot a space after the underscore?"
+        )
+
     BadOp op stack ->
       case op of
         HasType ->