Eric Seidel authored
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: #11049380b25ea
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