Allow CallStacks to be frozen

This introduces "freezing," an operation which prevents further
locations from being appended to a CallStack.  Library authors may want
to prevent CallStacks from exposing implementation details, as a matter
of hygiene. For example, in

```
head [] = error "head: empty list"

ghci> head []
*** Exception: head: empty list
CallStack (from implicit params):
  error, called at ...
```

including the call-site of `error` in `head` is not strictly necessary
as the error message already specifies clearly where the error came
from.

So we add a function `freezeCallStack` that wraps an existing CallStack,
preventing further call-sites from being pushed onto it. In other words,

```
pushCallStack callSite (freezeCallStack callStack) = freezeCallStack callStack
```

Now we can define `head` to not produce a CallStack at all

```
head [] =
  let ?callStack = freezeCallStack emptyCallStack
  in error "head: empty list"

ghci> head []
*** Exception: head: empty list
CallStack (from implicit params):
  error, called at ...
```

---

1. We add the `freezeCallStack` and `emptyCallStack` and update the
   definition of `CallStack` to support this functionality.

2. We add `errorWithoutStackTrace`, a variant of `error` that does not
   produce a stack trace, using this feature. I think this is a sensible
   wrapper function to provide in case users want it.

3. We replace uses of `error` in base with `errorWithoutStackTrace`. The
   rationale is that base does not export any functions that use CallStacks
   (except for `error` and `undefined`) so there's no way for the stack
   traces (from Implicit CallStacks) to include user-defined functions.
   They'll only contain the call to `error` itself. As base already has a
   good habit of providing useful error messages that name the triggering
   function, the stack trace really just adds noise to the error. (I don't
   have a strong opinion on whether we should include this third commit,
   but the change was very mechanical so I thought I'd include it anyway in
   case there's interest)

4. Updates tests in `array` and `stm` submodules

Test Plan: ./validate, new test is T11049

Reviewers: simonpj, nomeata, goldfire, austin, hvr, bgamari

Reviewed By: simonpj

Subscribers: thomie

Projects: #ghc

Differential Revision: https://phabricator.haskell.org/D1628

GHC Trac Issues: #11049

1 files changed, 3 insertions, 3 deletions

diff --git a/libraries/base/GHC/Event/Poll.hsc b/libraries/base/GHC/Event/Poll.hsc
index 6cbe14398b..b128572e71 100644
--- a/libraries/base/GHC/Event/Poll.hsc
+++ b/libraries/base/GHC/Event/Poll.hsc
@@ -17,7 +17,7 @@ import GHC.Base
 import qualified GHC.Event.Internal as E
 
 new :: IO E.Backend
-new = error "Poll back end not implemented for this platform"
+new = errorWithoutStackTrace "Poll back end not implemented for this platform"
 
 available :: Bool
 available = False
@@ -62,7 +62,7 @@ modifyFd p fd oevt nevt =
     return True
 
 modifyFdOnce :: Poll -> Fd -> E.Event -> IO Bool
-modifyFdOnce = error "modifyFdOnce not supported in Poll backend"
+modifyFdOnce = errorWithoutStackTrace "modifyFdOnce not supported in Poll backend"
 
 reworkFd :: Poll -> PollFd -> IO ()
 reworkFd p (PollFd fd npevt opevt) = do
@@ -72,7 +72,7 @@ reworkFd p (PollFd fd npevt opevt) = do
     else do
       found <- A.findIndex ((== fd) . pfdFd) ary
       case found of
-        Nothing        -> error "reworkFd: event not found"
+        Nothing        -> errorWithoutStackTrace "reworkFd: event not found"
         Just (i,_)
           | npevt /= 0 -> A.unsafeWrite ary i $ PollFd fd npevt 0
           | otherwise  -> A.removeAt ary i