[Haskell-cafe] type constructor confusion
westondan at imageworks.com
Thu Jun 19 14:42:20 EDT 2008
If it helps, feel free to use a different name for the data constructors
and their data type until the difference is painfully clear to you
(maybe suffix the constructor with a C or prefix by Mk).
Data types and constructors live in different namespaces and can happily
use the same identifier. That doesn't mean you have to, if it means more
work deciphering error messages.
Computers would be just as happy with identifiers like g2ch_Sw'K54h
(just take a look at the GHC simple core dump!), so pick ones that you
yourself find perspicuous.
Ryan Ingram wrote:
> It sounds like you need to split up your types a bit more.
> data HttpRequest = HttpRequest ...
> data HttpResponse = HttpResponse ...
> data HttpMessage = MsgRequest HttpRequest | MsgResponse HttpResponse
> -- alternatively
> -- type HttpMessage = Either HttpRequest HttpResponse
> Now you can have functions that take/return just an HttpRequest or
> just an HttpResponse, as well as functions that use either one via
> HttpMessage. In the latter case, you do need to pattern match to
> decide which one you have.
> -- ryan
> On 6/18/08, Stephen Howard <stephen at enterity.com> wrote:
>> Thanks Brandon, forgot to send my reply to the list:
>> Ok, so I am confusing things. Good to know. So my question is how do I
>> fulfill this scenario?
>> - I have an action that might return either an HttpResponse or an
>> HttpRequest, depending on if the IO in the action determined more work
>> needed doing. It's here, though I doubt it's "correct" yet:
>> requestHandler :: HttpRequest -> IO HttpResponse
>> requestHandler request = do
>> session <- sessionHandler request
>> ret <- uriHandler request
>> case ret of
>> HttpResponse -> ret
>> HttpRequest -> resourceHandler session ret
>> uriHandler :: HttpRequest -> IO HttpMessage
>> sessionHandler :: HttpRequest -> IO HttpSession
>> I've given the uriHandler a signature of IO HttpMessage because the
>> HttpMessage might be either an HttpResponse or an HttpRequest, and I don't
>> know how I should be specifying that. Ideas?
>> - Stephen
>> Brandon S. Allbery KF8NH wrote:
>>> On Jun 18, 2008, at 15:31 , Stephen Howard wrote:
>>>> HttpMessage.hs:36:20: Not in scope: type constructor or class
>>>> The troublesome line is the definition of the cookie function at the end
>> of the code. I've made
>>> Right. "HttpRequest" is a data constructor associated with the type
>> constructor "HttpMessage".
>>> (Data constructors are effectively functions; you used it in the context
>> of a type, not a function name.)
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