Error creating Haskell HTTP Request

Question

I am trying to create a Request value and am getting the following error:

ghci> :m +Network.HTTP Network.URI Data.Maybe
ghci> Request { rqURI = fromJust $ parseURI "http://www.google.com", rqMethod = GET, rqHeaders = [], rqBody = ""}

<interactive>:18:19:
    Couldn't match expected type `network-2.3.1.0:Network.URI.URI'
                with actual type `URI'
    In the `rqURI' field of a record
    In the expression:
      Request
        {rqURI = fromJust $ parseURI "http://www.google.com",
         rqMethod = GET, rqHeaders = [], rqBody = ""}
    In an equation for `it':
        it
          = Request
              {rqURI = fromJust $ parseURI "http://www.google.com",
               rqMethod = GET, rqHeaders = [], rqBody = ""}

cabal list network shows the following:

* network
    Synopsis: Low-level networking interface
    Default available version: 2.4.0.1
    Installed versions: 2.3.1.0, 2.4.0.1
    Homepage: https://github.com/haskell/network
    License:  BSD3

From the docs on Hackage, I believe that the URI I am creating with parseURI is a Network.URI.URI.

I am running the Haskell Platform 2012.4.0.0 (64 bit) on OS X Mountain Lion.

Is this an example of "cabal hell" that I hear about?

Daniel Fischer · Accepted Answer · 2012-12-16 14:31:11Z

3

Your HTTP package was built using network-2.3.1.0, but ghci loads the newest version of network unless it is explicitly told which version to use.

So parseURI returns a Maybe network-2.4.0.1:Network.URI.URI but the Request requires the type from network-2.3.1.0.

You can

invoke ghci with a -package flag specifying the version of network to use, $ ghci -package network-2.3.1.0 other arguments,
rebuild HTTP and everything depending on it against network-2.4.0.1 - in that case, it's probably better to rebuild everything that transitively depends on network-2.3.1.0.

Is this an example of "cabal hell" that I hear about?

I wouldn't go so far as to call that "hell" yet, but it's a small example of that indeed.

answered Dec 16, 2012 at 14:31

Daniel Fischer

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4 Comments

Ralph Over a year ago

The problem appears to be that hxt-9.3.1.1 depends specifically on network-2.3.1.0. Forcing a reinstall of network will break HXT.

Daniel Fischer Over a year ago

In that case, the -package flag is the way to go. Inconvenient it is, but it works.

Ralph Over a year ago

I actually got the original error compiling an application (actually running with runhaskell). I copied the relevant parts to GHCi to test it further and wrote my SOF question from that. Guess I'm out-of-luck.

Daniel Fischer Over a year ago

The -package flag is for GHC in general, not only ghci. If you want to compile a cabalised project, cabal figures out which version of network (in this example) gives a consistent plan and automatically chooses 2.3.1.0 (unless you use other things that depend on 2.4.0.1, in which case it'll tell you that it can't find a consistent plan). runhaskell -package ntework-2.3.1.0 whatever may work or not, I expect it would, but I'm not sure.

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