test: Add test for #301

If we use an arbitrary computation as the value for a DebugView field,
which is re-created from scratch everytime we recompute that DebugView
fields, then evaluating that thunk is not persisted across refreshing
the variable.

Every time we re-fresh the variable we re-create the thunk, so it makes
some sense that forcing it is not persisted across re-freshing, but
still a bit unsatisfactory (esp. since the variables are invalidated as
soon as a variable is forced, so we only see a flash of these values)

Marked as expect fail.

Tracked by #301

Author: alt-romes

test/haskell/Test/Integration/Variables.hs

@@ -8,9 +8,7 @@ import qualified Data.Text as T
 import Test.DAP
 import Test.Tasty
 import Test.Tasty.HUnit
-#ifdef mingw32_HOST_OS
 import Test.Tasty.ExpectedFailure
-#endif
 import DAP (Variable, variableValue)
 
 variableTests :: TestTree
@@ -33,6 +31,8 @@ variableTests =
     , testCase "hdv with containers (issue #47c)" hdvContainersDepTest
     , testCase "hdv in-memory with containers (issue #47d)" hdvContainersMemTest
     , testCase "hdv in-memory with text (issue #47e)" hdvTextMemTest
+    , expectFailBecause "issue #301" $
+      testCase "force thunk in custom DebugView field (issue #301)" thunkFieldTest
     ]
 
 intsAndStringsTest :: Assertion
@@ -319,3 +319,29 @@ hdvTextMemTest =
       action <- forceLazy (locals % "action")
       action @==? "\"this should be displayed as a simple string\""
       disconnect
+
+-- | A custom DebugView field that wraps a thunk, when forced, should return
+-- the forced value. A subsequent request should still show the result forced.
+-- It is no longer a thunk after being forced the first time!
+--
+-- Issue #301 observes that the subsequent request showed the field again as a
+-- thunk.
+thunkFieldTest :: Assertion
+thunkFieldTest =
+  withTestDAPServer "test/integration/T301" [] $ \test_dir server ->
+    withTestDAPServerClient server $ do
+      let cfg = mkLaunchConfig test_dir "Main.hs"
+      hitBreakpointWith cfg 21
+      locals <- fetchLocalVars
+      action <- forceLazy (locals % "action")
+      action @==? "T301-X"
+      ac <- expandVar action
+      ti <- forceLazy (ac % "thunkInt")
+      ti @==? "5050"
+
+      -- Re-request the parent and check the field is still evaluated.
+      -- (it does not need to be forced again!)
+      locals2 <- fetchLocalVars
+      ac2 <- expandVar (locals2 % "action")
+      (ac2 % "thunkInt") @==? "5050"
+      disconnect

test/integration/T301/Main.hs

@@ -0,0 +1,21 @@
+module Main where
+import GHC.Debugger.View.Class
+
+data X = X String
+    deriving Show
+
+instance DebugView X where
+    debugValue _ = simpleValue "T301-X" True
+    debugFields (X s) = pure $ VarFields
+        [ ("thunkInt", VarFieldValue intThunk)
+        ]
+      where
+        intThunk :: Int
+        intThunk = sum [1 .. length s + 99]
+
+main :: IO ()
+main = f (X "A")
+
+f :: Show a => a -> IO ()
+f action = do
+    print action

test/integration/T301/T301.cabal

@@ -0,0 +1,18 @@
+cabal-version:   3.14
+name:            T301
+version:         0.1.0.0
+license:         NONE
+author:          Rodrigo Mesquita
+maintainer:      rodrigo.m.mesquita@gmail.com
+build-type:      Simple
+
+common warnings
+    ghc-options: -Wall
+
+executable t301
+    import:           warnings
+    main-is:          Main.hs
+    build-depends:    base, containers
+    build-depends:    haskell-debugger-view
+    hs-source-dirs:   .
+    default-language: Haskell2010