#11049: Allow CallStacks to be hidden or cut -------------------------------------+------------------------------------- Reporter: nomeata | Owner: gridaphobe Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11035 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by gridaphobe): * owner: => gridaphobe -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11049#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11049: Allow CallStacks to be hidden or cut -------------------------------------+------------------------------------- Reporter: nomeata | Owner: gridaphobe Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11035 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nomeata): Sure. Assume we are given a function {{{#!hs readFile :: ?callstack :: CallStack => FilePath -> IO () }}} that prints a stack trace with every IO error that happens (`bar` in the description). Now assume we want to implement a function that uses the above, for example {{{#!hs readConfig :: IO Config readConfig = do s <- readFile "config.txt" -- line 42 return (parseConfig s) }}} and we deliberately do not want that function have a `?_::CallStack` constraint. What will happen when `config.txt` will be missing? `readFile` will raise an exception that includes a call stack that tells the user that this was raised due to `readFile` being called in line 42. But as the author of the `readConfig` function, I do not want this information (which is not very helpful to the user) to be omitted. This is the first use case. The second is related. I might now allow `readConfig` to have a `?_::CallStack` constraint, e.g. {{{#!hs readConfig :: ?callstack::CallStack => IO Config readConfig = do s <- readFile "config.txt" -- line 42 return (parseConfig s) }}} But I do want to provide a polished API, and not leak any information to the users of my API about my internals. So _do_ want the `?callstack` to be passed on to `readFile` and be included in the exception, but I _don’t_ want it to mention line 42; instead it should end with the (for the user relevant) information where `readConfig` was called. This is the second use case. So now to the suggested implementation: In both cases, I want to insert a marker into the callstack that makes the call stack printer ignore anything “below” or “after” it. This is the suggested `rootCallStack` value, and it allows me to write {{{#!hs readConfig :: IO Config readConfig = do s <- let ?callstack = rootCallStack in readFile "config.txt" -- line 42 return (parseConfig s) }}} resp. {{{#!hs readConfig :: ?callstack::CallStack => IO Config readConfig = do s <- let ?callstack = rootCallStack `pushCallStack` ?callstack in readFile "config.txt" -- line 42 return (parseConfig s) }}} to implement the above. The implementation sketch is ad-hoc, and there might be more elegant variants. This is not not necessarily an advocation of such “hiding internals”, but I think it should be possible. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11049#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11049: Allow CallStacks to be hidden or cut -------------------------------------+------------------------------------- Reporter: nomeata | Owner: gridaphobe Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11035 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): That's helpful. So we make public both `rootStack` and `pushCallStack`. And we allow implicit-parameter bindings for `?loc :: CallStack`. (I suppose that's always been possible, and you can use it already to install a `CallStack` from somewhere else. I guess that's an advantage.) Would you like to turn the description, and your comment:3, and any subsequent stuff, into a wiki page (perhaps the same page as the current `CallStack` one. Otherwise we have to mentally compose all these things in our heads. Thanks! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11049#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11049: Allow CallStacks to be hidden or cut -------------------------------------+------------------------------------- Reporter: nomeata | Owner: gridaphobe Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11035 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Let's initially have the overall design, movitation, examples, on a wiki page. That's what we usually do. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11049#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11049: Allow CallStacks to be hidden or cut -------------------------------------+------------------------------------- Reporter: nomeata | Owner: gridaphobe Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11035 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ben Gamari ): In [changeset:"380b25ea4754c2aea683538ffdb179f8946219a0/ghc" 380b25e/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="380b25ea4754c2aea683538ffdb179f8946219a0" 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 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11049#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11049: Allow CallStacks to be hidden or cut -------------------------------------+------------------------------------- Reporter: nomeata | Owner: gridaphobe Type: feature request | Status: closed Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11035 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * milestone: => 8.0.1 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11049#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler