Re: Case expressions in STG

Whoops, looks like my phone dropped Tom from the CC (fixed). Simon, what about the following: f = \x -> x g = \x -> (x,1) h = \x -> fst (g x) i = \x -> case f of f -> True _ -> False i f => True i h => ? If g isn't inlined into h and fst optimized out, wouldn't the head normal form of f and h be different, and the comparison fail even though it shouldn't? Or should it? I've taken function equality to be "f and g are equal iff f x == g x, for all x." Will

On Jun 18, 2014, at 7:04 PM, Simon Peyton Jones wrote:

I've forgotten what I intended in the STG paper, but GHC's Core language certainly allows case on a function; all it does is to force the function to head normal form.

Simon

| -----Original Message----- | From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of William | Knop | Sent: 18 June 2014 23:54 | To: ghc-devs@haskell.org | Subject: Re: Case expressions in STG | | Hi Tom, | | SPJ is surely more qualified to answer than I, but I'll take a stab. | | In general, it is computationally infeasible to compare functions. | Granted, in your example, the function isn't being compared-- and | therefore the case expr is extraneous. I don't think there exists a | feasible, uncontrived example, so I imagine that's why STG forbids it | (though I don't know where it's specified). | | Cheers, | Will | | | > On Jun 18, 2014, at 9:43 AM, Tom Ellis wrote: | > | > I am reading SPJ's seminal work "Implementing lazy functional languages | on | > stock hardware: the Spinless Tagless G-machine" (1992). The paper is | > available here | > | > http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.53.3729 | > | > On p62 we see "The expression scrutinised by a case expression must | > eventually evaluate either to a primitive value or a constructor | > application.". | > | > This doesn't make sense to me. Isn't the following a valid STG | program? | > What am I missing? Where is it specified that the scrutinee of a case | > expression cannot be of a function type? | > | > {x} \n {z} -> let f = \{x} \n {y} -> g x y | > in case f of f' -> f' z | > | > Thanks, | > | > Tom | > _______________________________________________ | > ghc-devs mailing list | > ghc-devs@haskell.org | > http://www.haskell.org/mailman/listinfo/ghc-devs | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | http://www.haskell.org/mailman/listinfo/ghc-devs