Add -fwarn-monomorphism-restriction (on by default) to warn when the MR is used

Users often trip up on the Dreaded Monomorphism Restriction.  This
warning flag tells you when the MR springs into action.

Currently it's on by default, but we could change that.

compiler/main/DynFlags.hs

@@ -148,6 +148,7 @@ data DynFlag
    | Opt_WarnOverlappingPatterns
    | Opt_WarnSimplePatterns
    | Opt_WarnTypeDefaults
+   | Opt_WarnMonomorphism
    | Opt_WarnUnusedBinds
    | Opt_WarnUnusedImports
    | Opt_WarnUnusedMatches
@@ -583,6 +584,7 @@ standardWarnings
 	Opt_WarnOverlappingPatterns,
 	Opt_WarnMissingFields,
 	Opt_WarnMissingMethods,
+	Opt_WarnMonomorphism,
 	Opt_WarnDuplicateExports
       ]
 
@@ -1023,6 +1025,7 @@ fFlags = [
   ( "warn-overlapping-patterns", 	Opt_WarnOverlappingPatterns ),
   ( "warn-simple-patterns",      	Opt_WarnSimplePatterns ),
   ( "warn-type-defaults",        	Opt_WarnTypeDefaults ),
+  ( "warn-monomorphism-restriction",   	Opt_WarnMonomorphism ),
   ( "warn-unused-binds",         	Opt_WarnUnusedBinds ),
   ( "warn-unused-imports",       	Opt_WarnUnusedImports ),
   ( "warn-unused-matches",       	Opt_WarnUnusedMatches ),

compiler/typecheck/TcRnMonad.lhs

@@ -732,7 +732,7 @@ addWarnTcM :: (TidyEnv, Message) -> TcM ()
 addWarnTcM (env0, msg)
  = do { ctxt <- getErrCtxt ;
 	ctxt_msgs <- do_ctxt env0 ctxt ;
-	addWarn (vcat (msg : ctxt_to_use ctxt_msgs)) }
+	addReport (vcat (ptext SLIT("Warning:") <+> msg : ctxt_to_use ctxt_msgs)) }
 
 warnTc :: Bool -> Message -> TcM ()
 warnTc warn_if_true warn_msg

compiler/typecheck/TcSimplify.lhs

@@ -1202,9 +1202,22 @@ tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
 	; gbl_tvs' <- tcGetGlobalTyVars
 	; constrained_dicts' <- mappM zonkInst constrained_dicts
 
-	; let constrained_tvs' = tyVarsOfInsts constrained_dicts'
-	      qtvs = (tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs')
-			 `minusVarSet` constrained_tvs'
+	; let qtvs1 = tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs'
+				-- As in tcSimplifyInfer
+
+		-- Do not quantify over constrained type variables:
+		-- this is the monomorphism restriction
+	      constrained_tvs' = tyVarsOfInsts constrained_dicts'
+	      qtvs = qtvs1 `minusVarSet` constrained_tvs'
+	      pp_bndrs = pprWithCommas (quotes . ppr) bndrs
+
+	-- Warn in the mono
+	; warn_mono <- doptM Opt_WarnMonomorphism
+	; warnTc (warn_mono && (constrained_tvs' `intersectsVarSet` qtvs1))
+		 (vcat[ ptext SLIT("the Monomorphism Restriction applies to the binding")
+				<> plural bndrs <+> ptext SLIT("for") <+> pp_bndrs,
+			ptext SLIT("Consider giving a type signature for") <+> pp_bndrs])
+
 	; traceTc (text "tcSimplifyRestricted" <+> vcat [
 		pprInsts wanteds, pprInsts constrained_dicts',
 		ppr _binds,

docs/users_guide/flags.xml

@@ -838,6 +838,13 @@
 	    <entry><option>-fno-warn-type-defaults</option></entry>
 	  </row>
 
+	  <row>
+	    <entry><option>-fwarn-monomorphism-restriction</option></entry>
+	    <entry>warn when the Monomorphism Restriction is applied</entry>
+	    <entry>dynamic</entry>
+	    <entry><option>-fno-warn-monomorphism-restriction</option></entry>
+	  </row>
+
 	  <row>
 	    <entry><option>-fwarn-unused-binds</option></entry>
 	    <entry>warn about bindings that are unused</entry>

docs/users_guide/using.xml

@@ -1135,6 +1135,20 @@ f "2"    = 2
 	</listitem>
       </varlistentry>
 
+      <varlistentry>
+	<term><option>-fwarn-monomorphism-restriction</option>:</term>
+	<listitem>
+	  <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
+	  <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
+	  <para>Have the compiler warn/inform you where in your source
+          the Haskell Monomorphism Restriction is applied.  If applied silently
+	  the MR can give rise to unexpected behaviour, so it can be helpful
+	  to have an explicit warning that it is being applied.</para>
+
+	  <para>This warning is on by default.</para>
+	</listitem>
+      </varlistentry>
+
       <varlistentry>
 	<term><option>-fwarn-unused-binds</option>:</term>
 	<listitem>