#9764: Home package modules silently override available modules from package database -------------------------------------+------------------------------------- Reporter: ezyang | Owner: ezyang Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.9 Component: Compiler | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: GHC | Related Tickets: accepts invalid program | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Description changed by ezyang: Old description:

To see this, define a module with the same name as some module in base. GHC will compile this:

{{{ module Data.List where foo = 2

module A where import Data.List x = print foo }}}

This behavior is intentional; the most compelling reason of behaving this way is that if you are calling `ghc` without `-hide-all-packages`, some random package you installed which just happened to provide a module name that conflicted with yours could cause your code to stop compiling if we treated the package database and home packages equally.

However, this behavior seems undesirable when `-hide-all-packages` is provided; at the very least, it seems like you might want to provide a warning that you're defining a module which conflicts with a module from a package you have exposed.

This behavior will be further undesirable when signatures come into the picture. Now, I will want to include a package containing a signature for `A`, and FURTHERMORE I may want to declare a signature `A.hsig` in the local package which augments this signature with some extra declarations I need. Under the current behavior, the externally imported signature is just ignored entirely, even though I wanted them to be merged together.

My proposal is to switch the behavior so that we don't prefer home modules if `-hide-all-packages` is provided. What's not great about this proposal is that it adds yet another discrepancy between bare GHC use and GHC use with `-hide-all-packages`. I'm interested to know what people would like.

New description: To see this, define a module with the same name as some module in base. GHC will compile this: {{{ module Data.List where foo = 2 module A where import Data.List x = print foo }}} This behavior is intentional; the most compelling reason of behaving this way is that if you are calling `ghc` without `-hide-all-packages`, some random package you installed which just happened to provide a module name that conflicted with yours could cause your code to stop compiling if we treated the package database and home packages equally. However, this behavior seems undesirable when `-hide-all-packages` is provided; at the very least, it seems like you might want to provide a warning that you're defining a module which conflicts with a module from a package you have exposed. This behavior will be further undesirable when signatures come into the picture. Now, I will want to include a package containing a signature for `A`, and FURTHERMORE I may want to declare a signature `A.hsig` in the local package which augments this signature with some extra declarations I need. Under the current behavior, the externally imported signature is just ignored entirely, even though I wanted them to be merged together. (A counter argument is that this is confusing for users, who will see the use of a declaration, browse to the local hsig file, and not see it defined; thus this mode of use should be disallowed.) My proposal is to switch the behavior so that we don't prefer home modules if `-hide-all-packages` is provided. What's not great about this proposal is that it adds yet another discrepancy between bare GHC use and GHC use with `-hide-all-packages`. I'm interested to know what people would like. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9764#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#9764: Home package modules silently override available modules from package database -------------------------------------+------------------------------------- Reporter: ezyang | Owner: ezyang Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.9 Component: Compiler | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: GHC | Related Tickets: accepts invalid program | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by ezyang): That is not what I was proposing, but it sounds much better! And I agree, in that case, there would be no discrepancies. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9764#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#9764: Home package modules silently override available modules from package database -------------------------------------+------------------------------------- Reporter: ezyang | Owner: ezyang Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.9 Component: Compiler | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: GHC | Related Tickets: accepts invalid program | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by ezyang): In GHC 7.10, there will be TWO ways to do the specific use-case you're asking for! The first is the new `-prelude-is` flag #9499, which will let you set Prelude to be a module not named Prelude. The second is package renaming #9375, which means you can include base but rename Prelude to another name BasePrelude. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9764#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler