fixing slow evaluation of displayable data (b/c it was printing)

src/IHaskell/Eval/Completion.hs

 {-# LANGUAGE NoImplicitPrelude, OverloadedStrings #-}
-{- | Description : generates tab-completion options
-
-  context-insensitive completion for what is probably
-  the identifier under the cursor.
-  
-  [@Known issues@]
-
-  > import Data.Lef<tab>
-  > System.IO.h<tab>
-  > Just.he<tab>
-
-  The first should not complete to Left. The second should only
-  include things like System.IO.hPutStrLn, not head. Qualified
-  names should not be confused by the third option.
+{- |
+Description : Generates tab completion options.
+
+This has a limited amount of context sensitivity. It distinguishes between four contexts at the moment:
+- import statements (completed using modules)
+- identifiers (completed using in scope values)
+- extensions via :ext (completed using GHC extensions)
+- qualified identifiers (completed using in-scope values)
 
 -}
 module IHaskell.Eval.Completion (complete, completionTarget, completionType, CompletionType(..)) where

src/IHaskell/Eval/Evaluate.hs

@@ -499,7 +499,7 @@ evalCommand output (Expression expr) state = do
       -- back gives very strange errors - all the types match but it
       -- refuses to decode back into a [DisplayData].
       -- Suppress output, so as not to mess up console.
-      capturedStatement (const $ return ()) displayExpr
+      out <- capturedStatement (const $ return ()) displayExpr
 
       displayedBytestring <- dynCompileExpr "IHaskell.Display.serializeDisplay it"
       case fromDynamic displayedBytestring of

src/IHaskell/Types.hs

@@ -301,7 +301,13 @@ instance Show ExecuteReplyStatus where
 data ExecutionState = Busy | Idle | Starting deriving Show
 
 -- | Data for display: a string with associated MIME type.
-data DisplayData = Display MimeType String deriving (Show, Typeable, Generic)
+data DisplayData = Display MimeType String deriving (Typeable, Generic)
+
+-- We can't print the actual data, otherwise this will be printed every
+-- time it gets computed because of the way the evaluator is structured.
+-- See how `displayExpr` is computed.
+instance Show DisplayData where
+  show _ = "Display"
 
 -- Allow DisplayData serialization
 instance Serialize DisplayData