Specific denotations for pure types
ganesh.sittampalam at credit-suisse.com
Sat Mar 21 11:52:41 EDT 2009
Are you proposing to ban all implementation-dependent behaviour
everywhere in Haskell? (Or perhaps relegate it all to IO?)
From: haskell-prime-bounces at haskell.org
[mailto:haskell-prime-bounces at haskell.org] On Behalf Of Conal Elliott
Sent: 21 March 2009 00:56
To: Achim Schneider
Cc: haskell-prime at haskell.org
Subject: Re: Specific denotations for pure types
yes, but dodgy isn't Bool, it's _a_ Bool.
Right. dodgy is _a_ Bool, and therefore its meaning is an element of
the meaning of Bool. If _any_ element of Bool (e.g. dodgy) has a
machine-dependent meaning, then the meaning of Bool itself much have a
complex enough structure to contain such an element.
On Fri, Mar 20, 2009 at 5:13 PM, Achim Schneider <barsoap at web.de> wrote:
Conal Elliott <conal at conal.net> wrote:
> big :: Int
> big = 2147483647
> dodgy :: Bool
> dodgy = big + 1 > big
> oops :: ()
> oops = if dodgy then () else undefined
> Assuming compositional semantics, the meaning of oops depends
> meaning of dodgy, which depends on the meaning of big+1, which
yes, but dodgy isn't Bool, it's _a_ Bool. You're worried about
semantics of (>) :: Int -> Int -> Bool, (+) :: Int -> Int -> Int
that forall n > 0 . x + n > x doesn't hold for Int. There are
infinitely many ways to get a Bool out of things that don't
be Int (not to mention infinitely many ways to get a Bool out of
Int in an architecture-independent manner), which makes me think
quite err... fundamentalistic to generalise that forall Bool .
MachineInfo -> Bool. In fact, if you can prove for a certain
MachineInfo -> ThatBool, you (most likely) just found a bug in
Shortly put: All that dodginess is fine with me, as long as it
the only way. Defaulting to machine-independent semantics at the
expense of performance would be a most sensible thing, and Conal
seems to think _way_ too abstractly.
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