Fix #354 by relaxing %expect to check for upper bound

Author: sgraf812

app/Main.lhs

@@ -165,15 +165,19 @@ Pretty print the AbsSyn.
 Report any conflicts in the grammar.
 
 >       case expect common_options of
->         Just n | n == sr && rr == 0 -> return ()
 >         Just _ | rr > 0 ->
 >                 die ("The grammar has reduce/reduce conflicts.\n" ++
 >                      "This is not allowed when an expect directive is given\n")
->         Just _ ->
+>         Just n | sr > n ->
 >                die ("The grammar has " ++ show sr ++
 >                     " shift/reduce conflicts.\n" ++
->                     "This is different from the number given in the " ++
->                     "expect directive\n")
+>                     "This is more than the " ++ show n ++
+>                     " given in the %expect directive\n")
+>         Just n | sr < n ->
+>                do hPutStrLn stderr ("shift/reduce conflicts:  " ++ show sr)
+>                   hPutStrLn stderr ("(the %expect directive permits up to " ++
+>                                     show n ++ ")")
+>         Just _ -> return ()
 >         _ -> do
 
 >          (if sr /= 0

doc/syntax.rst

@@ -255,11 +255,13 @@ These conflicts generate warnings.
 But when you have checked the warnings and made sure that Happy handles them correctly these warnings are just annoying.
 The ``%expect`` directive gives a way of avoiding them.
 Declaring ``%expect n`` is a way of telling Happy
-“There are exactly <n> shift/reduce conflicts and zero reduce/reduce conflicts in this grammar.
+“There are at most <n> shift/reduce conflicts and zero reduce/reduce conflicts in this grammar.
 I promise I have checked them and they are resolved correctly”.
 When processing the grammar, Happy will check the actual number of conflicts against the ``%expect`` declaration if any, and if there is a discrepancy then an error will be reported.
 
-Happy's ``%expect`` directive works exactly like that of yacc.
+Happy's ``%expect`` directive works like that of yacc, but relaxes the check to
+“at most <n> conflicts” instead of “exactly <n> conflicts” to allow for
+backward compatible improvements of the state machine.
 
 .. _sec-error-directive:
 

tests/Makefile

@@ -41,7 +41,7 @@ TESTS = Test.ly TestMulti.ly TestPrecedence.ly bug001.ly \
 	AttrGrammar001.y AttrGrammar002.y \
 	Pragma.y
 
-ERROR_TESTS = error001.y issue335.y
+ERROR_TESTS = error001.y expect_upper.y issue335.y
 
 # NOTE: `cabal` will set the `happy_datadir` env-var accordingly before invoking the test-suite
 #TEST_HAPPY_OPTS = --strict --template=..

tests/expect_upper.y

@@ -0,0 +1,36 @@
+-- Test that %expect checks for an upper bound, not an exact match.
+-- This grammar has exactly 1 shift/reduce conflict (the dangling else).
+-- %expect 2 should succeed because 1 <= 2.
+
+{
+module Main where
+}
+
+%expect 2
+%expect 1
+%name parse
+%tokentype { Token }
+
+%token
+  'if'   { If }
+  'then' { Then }
+  'else' { Else }
+  'x'    { X }
+
+%%
+
+stmt : 'if' 'x' 'then' stmt 'else' stmt  { $4 ++ $6 }
+     | 'if' 'x' 'then' stmt               { $4 }
+     | 'x'                                 { "x" }
+
+{
+main :: IO ()
+main =
+  if parse [If, X, Then, X] == "x"
+    then return ()
+    else error "bad parse"
+
+data Token = If | Then | Else | X
+
+happyError _ = error "parse error"
+}