Re: [Haskell-cafe] "show" for functional types

On Apr 5, 2006, at 12:42 PM, Josef Svenningsson wrote:

Sorry to barge in in the middle of your discussion here..

Hey, if we wanted a private conversation, we'd take it off-list. :-)

On 4/5/06, Robert Dockins wrote:

...

There is a fair bit of disagreement about what referential transparency means. I found the following link after googling around a bit; it seems to address some of these issues.

http://www.cs.indiana.edu/~sabry/papers/purelyFunctional.ps

Do you have any reference to the fact that there is any diagreement about the term? I know it has been used sloppily at times but I think it is pretty well defined. See section 4 of the following paper: http://www.dina.kvl.dk/~sestoft/papers/SondergaardSestoft1990.pdf

http://en.wikipedia.org/wiki/Referential_transparency http://lambda-the-ultimate.org/node/1237 It may be that experts have a well defined notion of what it means, but I think that non-experts (ie, most people) have a pretty vague idea of exactly what it means.

Readers digest: First we need a denotational semantics and a corresponding equality which I call '='. A language is then referentially transparent if for all expressions e,e1 and e2, if e1 = e2 then e[x:=e1] = e[x:=e2]. Here e[x:=e'] denotes substitution where the variable x is replaced with e' in the expression e.

So it's a standard substitutivity property. The only problem here is that Haskell has a pretty hairy denotational semantics and I don't think anyone has spelled it out in detail.

I think that may be the main problem, at least in this context. We can take a nice lovely definition of rt, as above, but its meaningless without a good semantic model. So we're forced to approximate and hand-wave, which is where the disagreement comes in.

The thing which I think comes closest is the following paper which investigates the denotational implications of have seq as a primitive: http://www.crab.rutgers.edu/~pjohann/seqFinal.pdf

Cheers,

/Josef

Thanks for these paper links; I'll be reading them as soon as I find a few moments. Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG