On Thu, Feb 3, 2011 at 2:03 PM, Luke Palmer
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. Cheers, Tim -- Tim Chevalier * http://cs.pdx.edu/~tjc/ * Often in error, never in doubt "an intelligent person fights for lost causes,realizing that others are merely effects" -- E.E. Cummings