[GHC] #11222: Teach strictness analysis about `catch`-like operations

14 Dec 2015

      #11222: Teach strictness analysis about `catch`-like operations
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider the `catch#` primop,

 {{{#!hs
 catch# :: (State# RealWorld -> (# State# RealWorld, a #) )      -- ^ thing
 to catch exceptions from
        -> (b -> State# RealWorld -> (# State# RealWorld, a #) ) -- ^
 exception handler
        -> State# RealWorld
        -> (# State# RealWorld, a #)
 }}}

 Semantically, this operation will always evaluate its first argument.
 Ideally we would indicate this in the primop's strictness signature in
 `primops.txt.pp`. Sadly, we can't do this at the moment due to a subtle
 wrinkle (discovered in #10712):

 Consider,

 {{{#!hs
 let r = \st -> raiseIO# blah st
 in catch (\st -> ...(r st)..) handler st
 }}}

 If we give the first argument of catch a strict signature, we'll get
 a demand `C(S)` for `r`; that is, `r` is definitely called with one
 argument, which indeed it is. The trouble comes when we feed `C(S)`
 into `r`'s RHS as the demand of the body as this will lead us to conclude
 that
 the whole `let` will diverge; clearly this isn't right.

 As Simon noted in ticket:10712#comment:4,
...
There's something very special about catch: it turns divergence into
 non-divergence.
In order to apply a proper strictness signature to `catch`-like operations
 we would need to teach the strictness analyzer about this property.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11222
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