[Haskell-cafe] Printing and Referential transparency excuse

[Cristian Baboi]

On Tue, 25 Dec 2007 11:05:59 +0200, Derek Elkins  
<derek.a.elkins at gmail.com> wrote:

> On Mon, 2007-12-24 at 11:15 +0200, Cristian Baboi wrote:

>> How can I define a function to do the inverse operation ?
>>    g :: String -> ( a -> b )
>>
>> This time I cannot see how referential transparency will deny it.
>> What's the excuse now ?
>
> Referential transparency has nothing to with this.  The problem here is
> parametricity.  However, if you give a non-parametric type (or add
> appropriate class constraints on a and b, or add (phantom) type
> information to the input), e.g. String -> (Int -> Int) then there is no
> trouble at all.
>

Thank you.

I finished reading the Haskell Language Report and I noticed that the  
class Show and Read are "toys".
I'll try to define some String -> (a -> b) functions some other time.

I wonder if parametricity is the only problem.
I guess that I'll have some problem with the arity of the result, but  
currying may help.