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, 4 insertions, 4 deletions

diff --git a/libraries/base/GHC/RTS/Flags.hsc b/libraries/base/GHC/RTS/Flags.hsc
index 62e720f7c0..12cead7821 100644
--- a/libraries/base/GHC/RTS/Flags.hsc
+++ b/libraries/base/GHC/RTS/Flags.hsc
@@ -87,7 +87,7 @@ instance Enum GiveGCStats where
     toEnum #{const ONELINE_GC_STATS} = OneLineGCStats
     toEnum #{const SUMMARY_GC_STATS} = SummaryGCStats
     toEnum #{const VERBOSE_GC_STATS} = VerboseGCStats
-    toEnum e = error ("invalid enum for GiveGCStats: " ++ show e)
+    toEnum e = errorWithoutStackTrace ("invalid enum for GiveGCStats: " ++ show e)
 
 -- | Parameters of the garbage collector.
 --
@@ -185,7 +185,7 @@ instance Enum DoCostCentres where
     toEnum #{const COST_CENTRES_VERBOSE} = CostCentresVerbose
     toEnum #{const COST_CENTRES_ALL}     = CostCentresAll
     toEnum #{const COST_CENTRES_XML}     = CostCentresXML
-    toEnum e = error ("invalid enum for DoCostCentres: " ++ show e)
+    toEnum e = errorWithoutStackTrace ("invalid enum for DoCostCentres: " ++ show e)
 
 -- | Parameters pertaining to the cost-center profiler.
 --
@@ -228,7 +228,7 @@ instance Enum DoHeapProfile where
     toEnum #{const HEAP_BY_RETAINER}     = HeapByRetainer
     toEnum #{const HEAP_BY_LDV}          = HeapByLDV
     toEnum #{const HEAP_BY_CLOSURE_TYPE} = HeapByClosureType
-    toEnum e = error ("invalid enum for DoHeapProfile: " ++ show e)
+    toEnum e = errorWithoutStackTrace ("invalid enum for DoHeapProfile: " ++ show e)
 
 -- | Parameters of the cost-center profiler
 --
@@ -267,7 +267,7 @@ instance Enum DoTrace where
     toEnum #{const TRACE_NONE}     = TraceNone
     toEnum #{const TRACE_EVENTLOG} = TraceEventLog
     toEnum #{const TRACE_STDERR}   = TraceStderr
-    toEnum e = error ("invalid enum for DoTrace: " ++ show e)
+    toEnum e = errorWithoutStackTrace ("invalid enum for DoTrace: " ++ show e)
 
 -- | Parameters pertaining to event tracing
 --