#13893: Improved Help for Control.Concurrent and Control.Exception when "variable not in scope" -------------------------------------+------------------------------------- Reporter: vanto | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by slyfox): Replying to [comment:5 vanto]:
Replying to [[span(style=color: #FF0000, slyfox )]]:\\
Why take preference of packages coming with ghc in this case?\\ Because these are the main libraries, of course. Those related to Haskell standard Prelude. (e.g Data.Char, Control.Applicative, Control.Monad, Data.Monoid, Data.Foldable and so on)\\\\
These functions are not in haskell's (nor in GHC's) Prelude module. Otherwise you would get a working program instead of errors about missing symbols. Let's pick a simple example: **fromList**. Which module(s) should GHC suggest for these?
It's not always the case that user wants to use base's primitives.\\ so what?\\\\
I would say suggesting incorrect module imports would lead to more confusion.\\ You do not trust the compiler?\\ Dr. Bjarne Stroustrup said that you should always trust the answers given by the compiler. And he's right. So that GHC gives the right answers, you have to program it in this way. it is obvious.\\ If the compiler is able to recognize the variables that belong to a module ( which I quote above) it must be able to suggest the appropriate module.\\ __And to go further if the compiler is able to suggest the right module it must be able to compile it independently of the programmer.__\\ The compiler is able to recognize many of the leading pragmas (file-
It meas that suggestion will not be correct and will mislead user to add wrong module into imports instead of stating the fact about missing import. header LANGUAGE pragmas).
Experiencing yourself by writing a few expressions that need their
You will be surprised to see how well the compiler is able to recognize
pragmas to work, but do not write them and compile the file. them and to propose them to the programmer to insert them in the program.
Even changes the order of the functions in the file. Moves the first function to the end for example and writes the function from end to beginning. Done with functions using various pragmas and you will be even more surprised.\\ __What the compiler can do is marvellous.__\\\\
Which procedure should define which packages (and modules) are blessed to deliver suggestions for symbols?\\ I do not quite understand the question.\\ But if you talk about the code that makes it work it would be the same
As ticket's subject is worded why Control.Concurrent and Control.Exception should be threated special?\\ Not at all. These modules are examples. The subject is badly defined in
In order to earn user's trust compiler should have minimum of false positives. Missing LANGUAGE pragma detection works differently from symbol resolution. GHC already knows all the syntax it can parse. The lexer branches guarded by LANGUAGE pragmas have conditional code of either parsing expressions correctly or reporting an error about missing LANGUAGE pragma. Even that simple idea and implementation does not always work correctly due to language extensions having a syntactic overlap. procedure which defines the pragmas I suppose or another choice that remains to be defined.\\\\ LANGUAGE pragma detection does not require loading modules not used by the program. Suggesting symbols would require such loading. The question is about list of such autoloaded modules. the ticket. Please reword the ticket description. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13893#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler