Fix Trac #3813: unused variables in GHCi bindings

[ghc-hetmet.git] / compiler / hsSyn / HsExpr.lhs

diff --git a/compiler/hsSyn/HsExpr.lhs b/compiler/hsSyn/HsExpr.lhs
index 6530ad6..10c106d 100644 (file)
--- a/compiler/hsSyn/HsExpr.lhs
+++ b/compiler/hsSyn/HsExpr.lhs
@@ -857,12 +857,9 @@ data StmtLR idL idR
      , recS_rec_ids :: [idR]   -- Ditto, but these variables are the "recursive" ones,
                               -- that are used before they are bound in the stmts of
                               -- the RecStmt. 
-
        -- An Id can be in both groups
        -- Both sets of Ids are (now) treated monomorphically
-       -- The only reason they are separate is becuase the DsArrows 
-       -- code uses them separately, and I don't understand it well
-       -- enough to change it
+       -- See Note [How RecStmt works] for why they are separate
 
        -- Rebindable syntax
      , recS_bind_fn :: SyntaxExpr idR -- The bind function
@@ -912,25 +909,30 @@ Array comprehensions are handled like list comprehensions -=chak
 Note [How RecStmt works]
 ~~~~~~~~~~~~~~~~~~~~~~~~
 Example:
-        HsDo [ BindStmt x ex
+   HsDo [ BindStmt x ex
 
-             , RecStmt [a::forall a. a -> a, b]
-                       [a::Int -> Int,       c]
-                       [ BindStmt b (return x)
-                       , LetStmt a = ea
-                       , BindStmt c ec ]
+        , RecStmt { recS_rec_ids   = [a, c]
+                  , recS_stmts            = [ BindStmt b (return (a,c))
+                                    , LetStmt a = ...b...
+                                    , BindStmt c ec ]
+                  , recS_later_ids = [a, b]
 
-             , return (a b) ]
+        , return (a b) ]
 
 Here, the RecStmt binds a,b,c; but
   - Only a,b are used in the stmts *following* the RecStmt,
-        This 'a' is *polymorphic'
   - Only a,c are used in the stmts *inside* the RecStmt
         *before* their bindings
-        This 'a' is monomorphic
 
-Nota Bene: the two a's have different types, even though they
-have the same Name.
+Why do we need *both* rec_ids and later_ids?  For monads they could be
+combined into a single set of variables, but not for arrows.  That
+follows from the types of the respective feedback operators:
+
+       mfix :: MonadFix m => (a -> m a) -> m a
+       loop :: ArrowLoop a => a (b,d) (c,d) -> a b c
+
+* For mfix, the 'a' covers the union of the later_ids and the rec_ids 
+* For 'loop', 'c' is the later_ids and 'd' is the rec_ids 
 
 Note [Typing a RecStmt]
 ~~~~~~~~~~~~~~~~~~~~~~~
@@ -976,7 +978,7 @@ pprDo DoExpr      stmts body = ptext (sLit "do")  <+> ppr_do_stmts stmts body
 pprDo (MDoExpr _) stmts body = ptext (sLit "mdo") <+> ppr_do_stmts stmts body
 pprDo ListComp    stmts body = pprComp brackets    stmts body
 pprDo PArrComp    stmts body = pprComp pa_brackets stmts body
-pprDo _           _     _    = panic "pprDo" -- PatGuard, ParStmtCxt
+pprDo _           _     _    = panic "pprDo" -- PatGuard, ParStmtCxt, GhciStmt
 
 ppr_do_stmts :: OutputableBndr id => [LStmt id] -> LHsExpr id -> SDoc
 -- Print a bunch of do stmts, with explicit braces and semicolons,
@@ -1090,6 +1092,7 @@ data HsMatchContext id  -- Context of a Match
 data HsStmtContext id
   = ListComp
   | DoExpr
+  | GhciStmt                            -- A command-line Stmt in GHCi pat <- rhs
   | MDoExpr PostTcTable                  -- Recursive do-expression
                                          -- (tiresomely, it needs table
                                          --  of its return/bind ops)
@@ -1141,6 +1144,7 @@ pprStmtContext (TransformStmtCtxt c)
  = sep [ptext (sLit "a transformed branch of"), pprStmtContext c]
 pprStmtContext (PatGuard ctxt)
  = ptext (sLit "a pattern guard for") $$ pprMatchContext ctxt
+pprStmtContext GhciStmt        = ptext (sLit "an interactive GHCi command")
 pprStmtContext DoExpr          = ptext (sLit "a 'do' expression")
 pprStmtContext (MDoExpr _)     = ptext (sLit "an 'mdo' expression")
 pprStmtContext ListComp        = ptext (sLit "a list comprehension")
@@ -1172,6 +1176,7 @@ matchContextErrString ProcExpr                   = ptext (sLit "proc")
 matchContextErrString (StmtCtxt (ParStmtCtxt c)) = matchContextErrString (StmtCtxt c)
 matchContextErrString (StmtCtxt (TransformStmtCtxt c)) = matchContextErrString (StmtCtxt c)
 matchContextErrString (StmtCtxt (PatGuard _))    = ptext (sLit "pattern guard")
+matchContextErrString (StmtCtxt GhciStmt)        = ptext (sLit "interactive GHCi command")
 matchContextErrString (StmtCtxt DoExpr)          = ptext (sLit "'do' expression")
 matchContextErrString (StmtCtxt (MDoExpr _))     = ptext (sLit "'mdo' expression")
 matchContextErrString (StmtCtxt ListComp)        = ptext (sLit "list comprehension")