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/codepages/MakeTable.hs b/libraries/base/codepages/MakeTable.hs
index 7b3328e2d8..394d447a6d 100644
--- a/libraries/base/codepages/MakeTable.hs
+++ b/libraries/base/codepages/MakeTable.hs
@@ -73,12 +73,12 @@ parseLine s = case words s of
 readHex' :: Enum a => String -> a
 readHex' ('0':'x':s) = case readHex s of
     [(n,"")] -> toEnum n -- explicitly call toEnum to catch overflow errors.
-    _ -> error $ "Can't read hex: " ++ show s
-readHex' s = error $ "Can't read hex: " ++ show s
+    _ -> errorWithoutStackTrace $ "Can't read hex: " ++ show s
+readHex' s = errorWithoutStackTrace $ "Can't read hex: " ++ show s
 
 readCharHex :: String -> Char
 readCharHex s = if c > fromEnum (maxBound :: Word16)
-                    then error "Can't handle non-BMP character."
+                    then errorWithoutStackTrace "Can't handle non-BMP character."
                     else toEnum c
     where c = readHex' s
 
@@ -255,7 +255,7 @@ showHex' s = "\\x" ++ showHex s ""
 
 repDualByte :: Enum c => c -> String
 repDualByte c
-    | n >= 2^(16::Int) = error "value is too high!"
+    | n >= 2^(16::Int) = errorWithoutStackTrace "value is too high!"
     -- NOTE : this assumes little-endian architecture.  But we're only using this on Windows,
     -- so it's probably OK.
     | otherwise = showHex' (n `mod` 256) ++ showHex' (n `div` 256)