Make Cryptol REPL accept input lines containing only comments.

Fixes #838.

src/Cryptol/Parser.y

@@ -371,6 +371,7 @@ decls_layout            :: { [Decl PName] }
 repl                    :: { ReplInput PName }
   : expr                   { ExprInput $1 }
   | let_decl               { LetInput $1 }
+  | {- empty -}            { EmptyInput }
 
 
 --------------------------------------------------------------------------------

src/Cryptol/Parser/AST.hs

@@ -258,10 +258,11 @@ data PrimType name = PrimType { primTName :: Located name
                               , primTFixity :: Maybe Fixity
                               } deriving (Show,Generic,NFData)
 
--- | Input at the REPL, which can either be an expression or a @let@
--- statement.
+-- | Input at the REPL, which can be an expression, a @let@
+-- statement, or empty (possibly a comment).
 data ReplInput name = ExprInput (Expr name)
                     | LetInput (Decl name)
+                    | EmptyInput
                       deriving (Eq, Show)
 
 -- | Export information for a declaration.

src/Cryptol/REPL/Command.hs

@@ -359,6 +359,9 @@ evalCmd str = do
       -- explicitly make this a top-level declaration, so that it will
       -- be generalized if mono-binds is enabled
       replEvalDecl decl
+    P.EmptyInput ->
+      -- comment or empty input does nothing
+      pure ()
 
 printCounterexample :: CounterExampleType -> P.Expr P.PName -> [Concrete.Value] -> REPL ()
 printCounterexample cexTy pexpr vs =