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/Real.hs b/libraries/base/GHC/Real.hs
index 31381d6bd9..186be27cdf 100644
--- a/libraries/base/GHC/Real.hs
+++ b/libraries/base/GHC/Real.hs
@@ -205,7 +205,7 @@ class  (Real a, Fractional a) => RealFrac a  where
                                 -1 -> n
                                 0  -> if even n then n else m
                                 1  -> m
-                                _  -> error "round default defn: Bad value"
+                                _  -> errorWithoutStackTrace "round default defn: Bad value"
 
     ceiling x           =  if r > 0 then n + 1 else n
                            where (n,r) = properFraction x
@@ -476,7 +476,7 @@ odd             =  not . even
         Int -> Int -> Int #-}
 {-# INLINABLE [1] (^) #-}    -- See Note [Inlining (^)]
 (^) :: (Num a, Integral b) => a -> b -> a
-x0 ^ y0 | y0 < 0    = error "Negative exponent"
+x0 ^ y0 | y0 < 0    = errorWithoutStackTrace "Negative exponent"
         | y0 == 0   = 1
         | otherwise = f x0 y0
     where -- f : x0 ^ y0 = x ^ y
@@ -585,7 +585,7 @@ x ^^ n          =  if n >= 0 then x^n else recip (x^(negate n))
 {-# RULES "(^)/Rational"    (^) = (^%^) #-}
 (^%^)           :: Integral a => Rational -> a -> Rational
 (n :% d) ^%^ e
-    | e < 0     = error "Negative exponent"
+    | e < 0     = errorWithoutStackTrace "Negative exponent"
     | e == 0    = 1 :% 1
     | otherwise = (n ^ e) :% (d ^ e)