#11035: Add implicit call-stacks to partial functions in base -------------------------------------+------------------------------------- Reporter: gridaphobe | Owner: 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: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nomeata):
So we can't just write `let ?callStack = emptyCallStack in error` because that's effectively what GHC is already doing for us. Instead we would need multiple versions of the callstack-aware functions, as Richard suggested.
What if we add a special `rootCallStack :: CallStack` which is a value such that {{{ x `pushCallStack` rootCallStack = rootCallStack }}} but {{{ rootCallStack `pushCallStack` x = rootCallStack “:” x }}} as before and a call stack that consists of only a rootCallStack causes no stack trace to be printed. This allows the following two important patterns: {{{ f x = let ?callStack = rootCallStack in {... something with error or any other callStack expecting function ...} }}} would prevent any CallStack information to appear in `{...}`, and {{{ f x = let ?callStack = rootCallStack `pushCallStack` ?callStack in {... something with error or any other callStack expecting function ...} }}} which would include a CallStack in exceptions raised in `{...}`, but the stack would end in `f`, and not expose any of the structure of the implementation. I’d like to have something like this for API hygiene and implementation hiding. But note that if we had that, constant propagation (or `rootCallStack`) and a specialization mechanism that’s very much like the existing type class constraint specialization would allow the compiler to specialize the implementation in the former case to one that does _not_ pass CallStacks around any more. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11035#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler