#10681: Teach GHC to interpret all hs files as two types of hs-boot files (abstract types only/full types + values) -------------------------------------+------------------------------------- Reporter: ezyang | Owner: ezyang Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 7.11 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- This is a new proposal for solving #1409. The big addition here is that we create **two** hs-boot files for each hs file: one that is a full hs-boot file to be imported by hs files to break loops, and a second which only includes abstract types for hs-boot files to import. C.f. #10679 **Discussion.** Here is a slightly goofy `hs-boot` file I've excerpted from GHC: {{{ module Packages where import {-# SOURCE #-} Module (PackageKey) import {-# SOURCE #-} DynFlags (DynFlags) packageKeyPackageIdString :: DynFlags -> PackageKey -> Maybe String }}} The `hs-boot` file must itself import `hs-boot` files, because this boot file is used by `Module` and `DynFlags`; without `{-# SOURCE #-}`, the boot file itself will participate in a cycle! But notice that there is something very interesting: a boot file is ONLY ever interested in importing other modules to get types. Never to import constructors or functions! We can use this observation to give us a mechanical transformation of an `hs` file to an `hs-boot` file, ASSUMING we can define a "second level" of `hs-boot` file to record our abstract types. **Example.** In this example, we have chosen to break the loop from `A`s import to `B`. {{{ module A where import {-# SOURCE #-} B data A = A B f :: A -> Bool f (A (B (A b))) = g b f _ = True module B where import A data B = B A g :: B -> Bool g (B (A (B b))) = f b g _ = False }}} The first-level `hs-boot`s are: {{{ module A where -- not actually used import {-# SOURCE 2 #-} B data A = A B f :: A -> Bool module B where import {-# SOURCE 2 #-} A data B = B A g :: B -> Bool }}} The second-level `hs-boot`s are: {{{ module A where data A module B where -- not actually used data B }}} **Commentary.** Here are some remarks: 1. Because we have to lift the transitive dependencies of anything we `{-# SOURCE #-}` import, it doesn't make sense to have a pragma which explicitly says what to put in the `hs-boot` file; instead, we just put in everything that we *can* handle in an `hs-boot` file (so exclude anything with missing type signatures, type families, etc.) Ideally, these automatic hs-boot files are generated lazily, but they should be reused as necessary. 2. This facility actually makes `{-# SOURCE #-}` a lot more attractive for increasing separate compilation: you can mark an import `{-# SOURCE #-}` to ensure that if its implementation changes, you don't have to recompile this module / you can build the module in parallel with that module. The downside is that when the imported file is modified, we have to regenerate the `hs-boot` stub before we conclude that the types have not changed (as opposed to with separate `hs-boot` files, where a modification to `hs` would not bump the timestamp on `hs-boot`. 3. This seems to definitely suggest that you should never need more than two levels of hs-boot nesting, or perhaps three with kinding. (But maybe someone has a fancy type system feature for which this is not true!) Maybe this applies to signature files too. 4. We can't force the first level of `hs-boot` files to be abstract types, for two reasons: (1) a source file importing the hs-boot file may really need the selector/constructor, and (2) the `hs-boot` files will reflect any cycles from the source files, that's no good! Rolling out to the second level breaks the cycle because abstract types never need any imports. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10681 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler