From a1579a34bba86590e3656e5c7e88a78a9fb2f584 Mon Sep 17 00:00:00 2001
From: Simon Marlow <simonmar@microsoft.com>
Date: Wed, 27 Sep 2006 13:25:50 +0000
Subject: [PATCH] Make printing of binding results optional in GHCi, and
 document it You can say :set -fno-print-bind-result in GHCi
 to disable this behaviour. Addresses #887

---
 compiler/main/DynFlags.hs         |    6 +++++-
 compiler/typecheck/TcRnDriver.lhs |   17 +++++++++++------
 docs/users_guide/6.6-notes.xml    |    9 +++++++++
 docs/users_guide/flags.xml        |    6 ++++++
 docs/users_guide/ghci.xml         |   10 ++++++++--
 5 files changed, 39 insertions(+), 9 deletions(-)

diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index 318df02..86598e9 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -195,6 +195,7 @@ data DynFlag
    | Opt_SplitObjs
    | Opt_StgStats
    | Opt_HideAllPackages
+   | Opt_PrintBindResult
 
    -- keeping stuff
    | Opt_KeepHiDiffs
@@ -433,6 +434,8 @@ defaultDynFlags =
     	    Opt_IgnoreInterfacePragmas,
     	    Opt_OmitInterfacePragmas
     
+	    -- on by default:
+	    Opt_PrintBindResult
                ] ++ standardWarnings,
                
         log_action = \severity srcSpan style msg -> 
@@ -1037,7 +1040,8 @@ fFlags = [
   ( "unbox-strict-fields",		Opt_UnboxStrictFields ),
   ( "dicts-cheap",			Opt_DictsCheap ),
   ( "excess-precision",			Opt_ExcessPrecision ),
-  ( "asm-mangling",			Opt_DoAsmMangling )
+  ( "asm-mangling",			Opt_DoAsmMangling ),
+  ( "print-bind-result",		Opt_PrintBindResult )
   ]
 
 glasgowExtsFlags = [ 
diff --git a/compiler/typecheck/TcRnDriver.lhs b/compiler/typecheck/TcRnDriver.lhs
index 875d4f6..acf003f 100644
--- a/compiler/typecheck/TcRnDriver.lhs
+++ b/compiler/typecheck/TcRnDriver.lhs
@@ -965,15 +965,20 @@ mkPlan stmt@(L loc (BindStmt {}))
   | [L _ v] <- collectLStmtBinders stmt		-- One binder, for a bind stmt 
   = do	{ let print_v  = L loc $ ExprStmt (nlHsApp (nlHsVar printName) (nlHsVar v))
 			          	   (HsVar thenIOName) placeHolderType
+
+	; print_bind_result <- doptM Opt_PrintBindResult
+	; let print_plan = do
+		  { stuff@([v_id], _) <- tcGhciStmts [stmt, print_v]
+		  ; v_ty <- zonkTcType (idType v_id)
+		  ; ifM (isUnitTy v_ty || not (isTauTy v_ty)) failM
+		  ; return stuff }
+
 	-- The plans are:
 	--	[stmt; print v]		but not if v::()
 	--	[stmt]
-	; runPlans [do { stuff@([v_id], _) <- tcGhciStmts [stmt, print_v]
-		       ; v_ty <- zonkTcType (idType v_id)
-		       ; ifM (isUnitTy v_ty || not (isTauTy v_ty)) failM
-		       ; return stuff },
-		    tcGhciStmts [stmt]
-	  ]}
+	; runPlans ((if print_bind_result then [print_plan] else []) ++
+		    [tcGhciStmts [stmt]])
+	}
 
 mkPlan stmt
   = tcGhciStmts [stmt]
diff --git a/docs/users_guide/6.6-notes.xml b/docs/users_guide/6.6-notes.xml
index 6b6df22..a45579b 100644
--- a/docs/users_guide/6.6-notes.xml
+++ b/docs/users_guide/6.6-notes.xml
@@ -370,6 +370,15 @@
           See <xref linkend="ghci-commands" /> for more information.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          GHCi now invokes <literal>print</literal> by default on the
+          result of IO actions and bindings at the prompt.  This is
+          occasionally not what you want, so it can be disabled (at
+          least for bindings) with 
+          <literal>:set -fno-print-bind-result</literal>.  See <xref
+          linkend="ghci-stmts" />.</para>
+      </listitem>
     </itemizedlist>
   </sect2>
 
diff --git a/docs/users_guide/flags.xml b/docs/users_guide/flags.xml
index b5c737a..d3d91b3 100644
--- a/docs/users_guide/flags.xml
+++ b/docs/users_guide/flags.xml
@@ -1874,6 +1874,12 @@
 	      <entry>dynamic</entry>
 	      <entry>-</entry>
 	    </row>
+	    <row>
+	      <entry><option>-fno-print-bind-result</option></entry>
+	      <entry><link id="ghci-stmts">Turn off printing of binding results in GHCi</link></entry>
+	      <entry>dynamic</entry>
+	      <entry>-</entry>
+	    </row>
 	  </tbody>
 	</tgroup>
       </informaltable>
diff --git a/docs/users_guide/ghci.xml b/docs/users_guide/ghci.xml
index 9ec07c3..ae55203 100644
--- a/docs/users_guide/ghci.xml
+++ b/docs/users_guide/ghci.xml
@@ -402,7 +402,7 @@ hello
 </screen>
 </para></sect2>
 
-    <sect2>
+    <sect2 id="ghci-stmts">
       <title>Using <literal>do-</literal>notation at the prompt</title>
       <indexterm><primary>do-notation</primary><secondary>in GHCi</secondary></indexterm>
       <indexterm><primary>statements</primary><secondary>in GHCi</secondary></indexterm>
@@ -444,7 +444,13 @@ Prelude>
 	      <literal>Show</literal></para>
 	  </listitem>
 	</itemizedlist>
-      </para>
+      The automatic printing of binding results can be supressed with
+      <option>:set -fno-print-bind-result</option> (this does not
+      supress printing the result of non-binding statements).
+      <indexterm><primary><option>-fno-print-bind-result</option></primary></indexterm><indexterm><primary><option>-fprint-bind-result</option></primary></indexterm>.
+      You might want to do this to prevent the result of binding
+      statements from being fully evaluated by the act of printing
+      them, for example.</para>
 
       <para>Of course, you can also bind normal non-IO expressions
       using the <literal>let</literal>-statement:</para>
-- 
1.7.10.4