[GHC] #10823: Don't mark functions with annotations as dead

[GHC]

#10823: Don't mark functions with annotations as dead
-------------------------------------+-------------------------------------
        Reporter:  spinda            |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Template Haskell  |                 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 Revisions:
-------------------------------------+-------------------------------------

Old description:

> A concrete example is in LiquidHaskell, which will soon function as a
> Core plugin. In LiquidHaskell, a subset of Haskell functions can be
> "lifted" to the logic level and used in types. For example:
>
> {{{
> module Test (ok) where
>
> [lq| inline gt |]
> gt x y = x > y
>
> [lq| ok :: x:Int -> { v:Int | gt x v } |]
> ok x = x + 1
> }}}
>
> {{{[lq| inline gt |]}}} generates an annotation on {{{gt}}}, which
> LiquidHaskell picks up at the plugin stage. It then tries to run a
> transformation from the {{{CoreBind}}} for {{{gt}}} to its internal
> representation of decidable logic. But since Core plugins are run after
> "dead" code is removed at the end of desugaring, and since {{{gt}}} is
> not exported, the {{{CoreBind}}} for {{{gt}}} does not reach the
> {{{CoreBinds}}} LH receives as a Core plugin.
>
> The solution for this is to prevent the desugarer from discarding
> {{{CoreBinds}}} for things with attached annotations.

New description:

 A concrete example is in LiquidHaskell, which will soon function as a Core
 plugin. In LiquidHaskell, a subset of Haskell functions can be "lifted" to
 the logic level and used in types. For example:

 {{{
 module Test (ok) where

 [lq| inline gt |]
 gt x y = x > y

 [lq| ok :: x:Int -> { v:Int | gt x v } |]
 ok x = x + 1
 }}}

 {{{[lq| inline gt |]}}} generates an annotation on {{{gt}}}, which
 LiquidHaskell picks up at the plugin stage. It then tries to run a
 transformation from the {{{CoreBind}}} for {{{gt}}} to its internal
 representation of decidable logic. But since Core plugins are run after
 "dead" code is removed at the end of desugaring, and since {{{gt}}} is not
 exported, the {{{CoreBind}}} for {{{gt}}} does not reach the
 {{{CoreBinds}}} LH receives as a Core plugin.

 The solution for this is to prevent the desugarer from discarding
 {{{CoreBinds}}} for things with attached annotations.

 ----

 Minimal example (without TH):

 {{{
 module AnnTest (thing) where

 {-# ANN thing False #-}
 thing :: Int
 thing = 7
 }}}

 {{{
 ➜  ghc -O0 -ddump-simpl AnnTest.hs
 [1 of 1] Compiling AnnTest          ( AnnTest.hs, AnnTest.o )

 ==================== Simplified expression ====================
 GHC.Desugar.toAnnotationWrapper
   @ GHC.Types.Bool Data.Data.$fDataBool GHC.Types.False



 ==================== Tidy Core ====================
 Result size of Tidy Core = {terms: 3, types: 1, coercions: 0}

 thing :: Int
 [GblId, Caf=NoCafRefs, Str=DmdType]
 thing = GHC.Types.I# 7
 }}}

 {{{
 module AnnTest () where

 {-# ANN thing False #-}
 thing :: Int
 thing = 7
 }}}

 {{{
 ➜  ghc -O0 -ddump-simpl AnnTest.hs
 [1 of 1] Compiling AnnTest          ( AnnTest.hs, AnnTest.o )

 ==================== Simplified expression ====================
 GHC.Desugar.toAnnotationWrapper
   @ GHC.Types.Bool Data.Data.$fDataBool GHC.Types.False



 ==================== Tidy Core ====================
 Result size of Tidy Core = {terms: 0, types: 0, coercions: 0}
 }}}

--

Comment (by spinda):

 Yes, it can be reproduced without TH. Minimal example attached to the
 ticket description.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10823#comment:10>
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