Add runStmt for executing IO statements and binding new names

AUTHORS

@@ -12,6 +12,7 @@ Daniel Wagner
 Evan Laforge
 Fernando Benavides
 Gwern Branwen
+Heinrich Apfelmus
 Jean Philippe Bernardy
 Jens Petersen
 Mark Wright

CHANGELOG.md

@@ -1,3 +1,7 @@
+### Upcoming release
+
+* Add `runStmt` for executing statements in the `IO` monad and binding new names.
+
 ### 0.7.0
 
 * Support for GHC 8.2

examples/example.hs

@@ -68,3 +68,10 @@ testHint =
       say "Here we evaluate an expression of type string, that when evaluated (again) leads to a string"
       res <- interpret "head $ map show [\"Worked!\", \"Didn't work\"]" infer >>= flip interpret infer
       say res
+      emptyLine
+      say "We can also execute statements in the IO monad and bind new names, e.g."
+      let stmts = ["x <- return 42", "print x"]
+      forM_ stmts $ \s -> do
+          say $ "    " ++ s
+          runStmt s
+      emptyLine

src/Hint/Eval.hs

@@ -1,11 +1,14 @@
 module Hint.Eval (
       interpret, as, infer,
       unsafeInterpret,
-      eval, parens
+      eval, runStmt,
+      parens
 ) where
 
 import qualified GHC.Exts (unsafeCoerce#)
 
+import Control.Exception
+
 import Data.Typeable hiding (typeOf)
 import qualified Data.Typeable (typeOf)
 
@@ -55,6 +58,31 @@ eval expr = do in_scope_show   <- supportShow
                let show_expr = unwords [in_scope_show, parens expr]
                unsafeInterpret show_expr in_scope_String
 
+-- | Evaluate a statement in the 'IO' monad, possibly binding new names.
+--
+-- Example:
+--
+-- > runStmt "x <- return 42"
+-- > runStmt "print x"
+runStmt :: (MonadInterpreter m) => String -> m ()
+runStmt = mayFail . runGhc1 go
+    where
+#if __GLASGOW_HASKELL__ >= 800
+    go statements = do
+        result <- GHC.execStmt statements GHC.execOptions
+        return $ case result of
+            GHC.ExecComplete { GHC.execResult = Right _ } -> Just ()
+            GHC.ExecComplete { GHC.execResult = Left  e } -> throw e
+            _                                             -> Nothing
+#else
+    go statements = do
+        result <- GHC.runStmt statements GHC.RunToCompletion
+        return $ case result of
+            GHC.RunOk _        -> Just ()
+            GHC.RunException e -> throw e
+            _                  -> Nothing
+#endif
+
 -- | Conceptually, @parens s = \"(\" ++ s ++ \")\"@, where s is any valid haskell
 -- expression. In practice, it is harder than this.
 -- Observe that if @s@ ends with a trailing comment, then @parens s@ would

src/Language/Haskell/Interpreter.hs

@@ -36,7 +36,7 @@ module Language.Haskell.Interpreter(
     -- ** Type inference
      typeOf, typeChecks, kindOf, normalizeType,
     -- ** Evaluation
-     interpret, as, infer, eval,
+     interpret, as, infer, eval, runStmt,
     -- * Error handling
      InterpreterError(..), GhcError(..), MultipleInstancesNotAllowed(..),
     -- * Miscellaneous