Case expressions in STG

[Simon Peyton Jones]

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 at haskell.org] On Behalf Of William
| Knop
| Sent: 18 June 2014 23:54
| To: ghc-devs at 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 <tom-lists-ghc-
| devs at jaguarpaw.co.uk> 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
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