Refactor FFI error messages

This patch was provoked by Trac #5610, which I finally got a moment to look at.

In the end I added a new data type ErrUtils.Validity,

  data Validity
    = IsValid            -- Everything is fine
    | NotValid MsgDoc    -- A problem, and some indication of why

with some suitable combinators, and used it where appropriate (which touches
quite a few modules).  The main payoff is that error messages improve for
FFI type validation.

8 files changed, 43 insertions, 23 deletions

diff --git a/testsuite/tests/ffi/should_fail/T3066.stderr b/testsuite/tests/ffi/should_fail/T3066.stderr
index 2bbaf629c7..e6d292d4ec 100644
--- a/testsuite/tests/ffi/should_fail/T3066.stderr
+++ b/testsuite/tests/ffi/should_fail/T3066.stderr
@@ -1,6 +1,7 @@
 
 T3066.hs:6:1:
-    Unacceptable argument type in foreign declaration: forall u. Ptr ()
+    Unacceptable argument type in foreign declaration:
+      ‘forall u. Ptr ()’ is not a data type
     When checking declaration:
-        foreign import ccall safe "static bla" bla
-          :: (forall u. X u) -> IO ()
+      foreign import ccall safe "static bla" bla
+        :: (forall u. X u) -> IO ()
diff --git a/testsuite/tests/ffi/should_fail/T5664.stderr b/testsuite/tests/ffi/should_fail/T5664.stderr
index 30bd017a1d..c1652c2288 100644
--- a/testsuite/tests/ffi/should_fail/T5664.stderr
+++ b/testsuite/tests/ffi/should_fail/T5664.stderr
@@ -1,13 +1,16 @@
 
 T5664.hs:15:1:
     Unacceptable argument type in foreign declaration:
-      FunPtr (D -> IO ())
+      Expected: Ptr/FunPtr (Int32 -> IO ()),
+        Actual: FunPtr (D -> IO ())
     When checking declaration:
       foreign import ccall safe "dynamic" mkFun3
         :: FunPtr (D -> IO ()) -> CInt -> IO ()
 
 T5664.hs:24:1:
-    Unacceptable result type in foreign declaration: IO (FunPtr (IO D))
+    Unacceptable result type in foreign declaration:
+      Expected: Ptr/FunPtr (IO Int32),
+        Actual: FunPtr (IO D)
     When checking declaration:
       foreign import ccall safe "wrapper" mkCallBack3
         :: IO CInt -> IO (FunPtr (IO D))
diff --git a/testsuite/tests/ffi/should_fail/T7506.stderr b/testsuite/tests/ffi/should_fail/T7506.stderr
index e8e95a9275..dd893df155 100644
--- a/testsuite/tests/ffi/should_fail/T7506.stderr
+++ b/testsuite/tests/ffi/should_fail/T7506.stderr
@@ -1,7 +1,8 @@
 
 T7506.hs:6:1:
-    Unacceptable type in foreign declaration: Int -> IO ()
-    A foreign-imported address (via &foo) must have type (Ptr a) or (FunPtr a)
+    Unacceptable type in foreign declaration:
+      ‘Int -> IO ()’ cannot be marshalled in a foreign call
+      A foreign-imported address (via &foo) must have type (Ptr a) or (FunPtr a)
     When checking declaration:
       foreign import ccall safe "static stdio.h &putchar" c_putchar
         :: Int -> IO ()
diff --git a/testsuite/tests/ffi/should_fail/ccfail001.stderr b/testsuite/tests/ffi/should_fail/ccfail001.stderr
index 813c5d187c..e890041b02 100644
--- a/testsuite/tests/ffi/should_fail/ccfail001.stderr
+++ b/testsuite/tests/ffi/should_fail/ccfail001.stderr
@@ -1,6 +1,7 @@
 
 ccfail001.hs:10:1:
-    Unacceptable result type in foreign declaration: State# RealWorld
+    Unacceptable result type in foreign declaration:
+      ‘State# RealWorld’ cannot be marshalled in a foreign call
     When checking declaration:
-        foreign import ccall safe "static foo" foo
-          :: Int -> State# RealWorld
+      foreign import ccall safe "static foo" foo
+        :: Int -> State# RealWorld
diff --git a/testsuite/tests/ffi/should_fail/ccfail002.stderr b/testsuite/tests/ffi/should_fail/ccfail002.stderr
index dfff4272cd..309fa521d2 100644
--- a/testsuite/tests/ffi/should_fail/ccfail002.stderr
+++ b/testsuite/tests/ffi/should_fail/ccfail002.stderr
@@ -1,7 +1,7 @@
 
 ccfail002.hs:10:1:
     Unacceptable result type in foreign declaration:
-        (# Int#, Int#, Int# #)
+      ‘(# Int#, Int#, Int# #)’ cannot be marshalled in a foreign call
     When checking declaration:
-        foreign import ccall unsafe "static foo" foo
-          :: Int# -> Int# -> Int# -> (# Int#, Int#, Int# #)
+      foreign import ccall unsafe "static foo" foo
+        :: Int# -> Int# -> Int# -> (# Int#, Int#, Int# #)
diff --git a/testsuite/tests/ffi/should_fail/ccfail003.stderr b/testsuite/tests/ffi/should_fail/ccfail003.stderr
index 4ce9db572d..6afdd7678f 100644
--- a/testsuite/tests/ffi/should_fail/ccfail003.stderr
+++ b/testsuite/tests/ffi/should_fail/ccfail003.stderr
@@ -1,10 +1,12 @@
 
 ccfail003.hs:7:1:
-    Unacceptable argument type in foreign declaration: Int#
+    Unacceptable argument type in foreign declaration:
+      ‘Int#’ cannot be marshalled in a foreign call
     When checking declaration:
       foreign export ccall "foo" foo :: Int# -> IO ()
 
 ccfail003.hs:10:1:
-    Unacceptable result type in foreign declaration: Int#
+    Unacceptable result type in foreign declaration:
+      ‘Int#’ cannot be marshalled in a foreign call
     When checking declaration:
       foreign export ccall "bar" bar :: Int -> Int#
diff --git a/testsuite/tests/ffi/should_fail/ccfail004.stderr b/testsuite/tests/ffi/should_fail/ccfail004.stderr
index cce4258911..f54ac91aa3 100644
--- a/testsuite/tests/ffi/should_fail/ccfail004.stderr
+++ b/testsuite/tests/ffi/should_fail/ccfail004.stderr
@@ -1,26 +1,36 @@
 
-
 ccfail004.hs:9:1:
-    Unacceptable argument type in foreign declaration: NInt
+    Unacceptable argument type in foreign declaration:
+      ‘NInt’ cannot be marshalled in a foreign call
+        because its data construtor is not in scope
+        Possible fix: import the data constructor to bring it into scope
     When checking declaration:
       foreign import ccall safe "static f1" f1 :: NInt -> IO Int
 
 ccfail004.hs:10:1:
-    Unacceptable result type in foreign declaration: IO NInt
+    Unacceptable result type in foreign declaration:
+      ‘NInt’ cannot be marshalled in a foreign call
+        because its data construtor is not in scope
+        Possible fix: import the data constructor to bring it into scope
     When checking declaration:
       foreign import ccall safe "static f2" f2 :: Int -> IO NInt
 
 ccfail004.hs:11:1:
-    Unacceptable result type in foreign declaration: NIO Int
+    Unacceptable result type in foreign declaration:
+      ‘NIO Int’ cannot be marshalled in a foreign call
+        because the data construtor for ‘NIO’ is not in scope
+        Possible fix: import the data constructor to bring it into scope
     When checking declaration:
       foreign import ccall safe "static f3" f3 :: Int -> NIO Int
 
 ccfail004.hs:14:1:
-    Unacceptable argument type in foreign declaration: [NT]
+    Unacceptable argument type in foreign declaration:
+      ‘[NT]’ cannot be marshalled in a foreign call
     When checking declaration:
       foreign import ccall safe "static f4" f4 :: NT -> IO ()
 
 ccfail004.hs:15:1:
-    Unacceptable result type in foreign declaration: IO [NT]
+    Unacceptable result type in foreign declaration:
+      ‘[NT]’ cannot be marshalled in a foreign call
     When checking declaration:
       foreign import ccall safe "static f5" f5 :: IO NT
diff --git a/testsuite/tests/ffi/should_fail/ccfail005.stderr b/testsuite/tests/ffi/should_fail/ccfail005.stderr
index 0d96fe91e3..413faa702c 100644
--- a/testsuite/tests/ffi/should_fail/ccfail005.stderr
+++ b/testsuite/tests/ffi/should_fail/ccfail005.stderr
@@ -1,10 +1,12 @@
 
 ccfail005.hs:14:1:
-    Unacceptable argument type in foreign declaration: D
+    Unacceptable argument type in foreign declaration:
+      ‘D’ cannot be marshalled in a foreign call
     When checking declaration:
       foreign import ccall safe "static f1" f1 :: F Bool
 
 ccfail005.hs:15:1:
-    Unacceptable result type in foreign declaration: IO D
+    Unacceptable result type in foreign declaration:
+      ‘D’ cannot be marshalled in a foreign call
     When checking declaration:
       foreign import ccall safe "static f2" f2 :: F Char