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

Dan Doel dan.doel at gmail.com
Thu Feb 3 23:42:49 CET 2011

On Thursday 03 February 2011 5:12:54 PM Tim Chevalier wrote:
> On Thu, Feb 3, 2011 at 2:03 PM, Luke Palmer <lrpalmer at gmail.com> 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
> Yes, and you can see this in the Core code that Don posted: in version
> (A), GHC optimized away the outer call to error. But in version (B),
> the demand analyzer only knows that ($) is strict in its first
> argument -- it's not strict in its second. So it's not obviously safe
> to do the same optimization: the demand analyzer doesn't "look
> through" higher-order function arguments IIRC. (You can confirm this
> for yourself if you also want to read the demand analyzer output.)
> If ($) were getting inlined, the code would look the same coming into
> demand analysis in both cases, so you wouldn't see a difference. So
> I'm guessing you're compiling with -O0.

Whatever is going on, it has to be active during ghci, because all these 
differences can be seen during interpretation (in 7.0.1, at least).

  Prelude> error (error "foo")
  *** Exception: foo
  Prelude> error $ error "foo"
  *** Exception: *** Exception: foo
  Prelude> let g :: (a -> b) -> a -> b ; g f x = f x in g error (error "foo")
  *** Exception: foo
  Prelude> let g :: (a -> b) -> a -> b ; g f x = f x
  Prelude> g error (error "foo")
  *** Exception: *** Exception: foo
  Prelude> let foo = error "foo" in error foo
  *** Exception: foo
  Prelude> let foo = error "foo"
  Prelude> error foo
  *** Exception: *** Exception: foo

Actually compiling seems to remove the difference in 7.0.1, at least, because 
the output is always:

  Foo: foo

regardless of ($) or not ('fix error' hangs without output as well, which 
isn't what I thought would happen).

Anyhow, that rules out most general-purpose optimizations (including 
strictness analysis, I thought).

- Dan

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