[Haskell-cafe] ($) not as transparent as it seems

[Steffen Schuldenzucker]

Thanks to all of you for making GHC's behaviour yet a bit clearer to me.

On 02/03/2011 11:25 PM, Daniel Fischer wrote:
> On Thursday 03 February 2011 23:03:36, Luke Palmer wrote:
>    
>> This is probably a result of strictness analysis.  error is
>> technically strict, so it is reasonable to optimize to:
>>
>>      let e = error "foo" in e `seq` error e
>>
>>      
> I think so too.
> Unoptimised,
>
> module Errors where
>
> foo = error (error "foo")
>
> bar = error $ error "bar"
>
> produces the core
> ========================================
> Errors.bar :: forall a_aaN. a_aaN
> [GblId]
> Errors.bar =
>    \ (@ a_aaN) ->
>      GHC.Base.$
>        @ [GHC.Types.Char]
>        @ a_aaN
>        (GHC.Err.error @ a_aaN)
>        (GHC.Err.error @ [GHC.Types.Char] (GHC.Base.unpackCString# "bar"))
>
> a_rb8 :: [GHC.Types.Char]
> [GblId, Str=DmdType b]
> a_rb8 =
>    GHC.Err.error @ [GHC.Types.Char] (GHC.Base.unpackCString# "foo")
>
> Errors.foo :: forall a_aaP. a_aaP
> [GblId]
> Errors.foo =
>    (\ (@ a_aaP) ->  a_rb8)
>    `cast` (forall a_aaP. CoUnsafe [GHC.Types.Char] a_aaP
>            :: (forall a_aaP. [GHC.Types.Char]) ~ (forall a_aaP. a_aaP))
> ==============================================
>
> The first argument to ($) is evaluated before the second [because the
> function may be lazy), resulting in the start of the error message
> "***Exception: ", then that error-call must evaluate its argument, error
> "bar", which results in "***Exception: bar" (and terminates the thread) and
> two "***Exception: " being printed. If I interpret the core correctly,
> error is so well known to the compiler that it strips off the outer `error'
> in foo even without optimisations (which surprises me a bit).
>
> With optimisations, ($) is inlined and `error $ error "bar"' is transformed
> to error (error "bar"), from then on both have identical structure and
> arrive at (mutatis mutandis) the same core (which is nearly the same as foo
> got without optimisations).
>