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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
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Summary:
The idea here is that this gives a more detailed stack trace in two
cases:
1. With `-prof` and `-fprof-auto`
2. In GHCi (see #11047)
Example, with an error inserted in nofib/shootout/binary-trees:
```
$ ./Main 3
Main: z
CallStack (from ImplicitParams):
error, called at Main.hs:67:29 in main:Main
CallStack (from -prof):
Main.check' (Main.hs:(67,1)-(68,82))
Main.check (Main.hs:63:1-21)
Main.stretch (Main.hs:32:35-57)
Main.main.c (Main.hs:32:9-57)
Main.main (Main.hs:(27,1)-(43,42))
Main.CAF (<entire-module>)
```
This doesn't quite obsolete +RTS -xc, which also attempts to display
more information in the case when the error is in a CAF, but I'm
exploring other solutions to that.
Includes submodule updates.
Test Plan: validate
Reviewers: simonpj, ezyang, gridaphobe, bgamari, hvr, austin
Reviewed By: bgamari
Subscribers: thomie
Differential Revision: https://phabricator.haskell.org/D1426
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The default top-level exception handler now uses the `Show` instance for
`ErrorCall` when printing exceptions, so it will actually print the out-of-band
data (e.g. `CallStack`s) in compiled binaries, instead of just printing the
error message.
This also updates the hpc submodule to fix the test output.
Reviewed By: austin, thomie
Differential Revision: https://phabricator.haskell.org/D1217
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