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.
Dan
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 <[EMAIL PROTECTED]> 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
`HttpRequest'
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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