Function definition in GHCi

This patch allows define and re-define functions in ghci. `let` is not
required anymore (but can be used).

Idea: If ghci input string can be parsed as statement then run it as
statement else run it as declaration.

Reviewers: mpickering, bgamari, austin

Reviewed By: mpickering, bgamari, austin

Subscribers: hvr, mpickering, dterei, thomie

Differential Revision: https://phabricator.haskell.org/D1299

GHC Trac Issues: #7253

2 files changed, 23 insertions, 21 deletions

diff --git a/ghc/GhciMonad.hs b/ghc/GhciMonad.hs
index 7dd005b99e..6d068be485 100644
--- a/ghc/GhciMonad.hs
+++ b/ghc/GhciMonad.hs
@@ -19,7 +19,7 @@ module GhciMonad (
         TickArray,
         getDynFlags,
 
-        runStmt, runDecls, resume, timeIt, recordBreak, revertCAFs,
+        isStmt, runStmt, runDecls, resume, timeIt, recordBreak, revertCAFs,
 
         printForUser, printForUserPartWay, prettyLocations,
         initInterpBuffering, turnOffBuffering, flushInterpBuffers,
@@ -50,6 +50,10 @@ import System.IO
 import Control.Monad
 import GHC.Exts
 
+import qualified Lexer (ParseResult(..), unP, mkPState)
+import qualified Parser (parseStmt)
+import StringBuffer (stringToStringBuffer)
+
 import System.Console.Haskeline (CompletionFunc, InputT)
 import qualified System.Console.Haskeline as Haskeline
 import Control.Monad.Trans.Class
@@ -262,6 +266,19 @@ printForUserPartWay doc = do
   dflags <- getDynFlags
   liftIO $ Outputable.printForUserPartWay dflags stdout (pprUserLength dflags) unqual doc
 
+isStmt :: String -> GHCi Bool
+isStmt stmt = do
+  st <- getGHCiState
+  dflags <- GHC.getInteractiveDynFlags
+
+  let buf = stringToStringBuffer stmt
+      loc = mkRealSrcLoc (fsLit "<interactive>") (line_number st) 1
+      parser = Parser.parseStmt
+
+  case Lexer.unP parser (Lexer.mkPState dflags buf loc) of
+    Lexer.POk _ _ -> return True
+    Lexer.PFailed _ _ -> return False
+
 -- | Run a single Haskell expression
 runStmt :: String -> GHC.SingleStep -> GHCi (Maybe GHC.ExecResult)
 runStmt expr step = do
diff --git a/ghc/InteractiveUI.hs b/ghc/InteractiveUI.hs
index 8f861eecac..e5c4e11dea 100644
--- a/ghc/InteractiveUI.hs
+++ b/ghc/InteractiveUI.hs
@@ -897,22 +897,6 @@ enqueueCommands cmds = do
   cmds `deepseq` return ()
   modifyGHCiState $ \st -> st{ cmdqueue = cmds ++ cmdqueue st }
 
--- | If we one of these strings prefixes a command, then we treat it as a decl
--- rather than a stmt. NB that the appropriate decl prefixes depends on the
--- flag settings (Trac #9915)
-declPrefixes :: DynFlags -> [String]
-declPrefixes dflags = keywords ++ concat opt_keywords
-  where
-    keywords = [ "class ", "instance "
-               , "data ", "newtype ", "type "
-               , "default ", "default("
-               ]
-
-    opt_keywords = [ ["foreign "  | xopt Opt_ForeignFunctionInterface dflags]
-                   , ["deriving " | xopt Opt_StandaloneDeriving dflags]
-                   , ["pattern "  | xopt Opt_PatternSynonyms dflags]
-                   ]
-
 -- | Entry point to execute some haskell code from user.
 -- The return value True indicates success, as in `runOneCommand`.
 runStmt :: String -> SingleStep -> GHCi (Maybe GHC.ExecResult)
@@ -927,10 +911,11 @@ runStmt stmt step
  = do addImportToContext stmt; return (Just (GHC.ExecComplete (Right []) 0))
 
  | otherwise
- = do dflags <- getDynFlags
-      if any (stmt `looks_like`) (declPrefixes dflags)
-        then run_decl
-        else run_stmt
+ = do
+     parse_res <- GhciMonad.isStmt stmt
+     if parse_res
+       then run_stmt
+       else run_decl
   where
     run_decl =
         do _ <- liftIO $ tryIO $ hFlushAll stdin