4

I am trying to write a simple script that takes as input a URL (or set of URLs) and as output it downloads the contents of that page to a file (in particular I am trying to download hundreds of JSON files, which ultimately I wish to diff against other JSON files).

In a file, download.hs, I have import "HTTP" Network.HTTP.

When I run: $ ghc -o download download.hs

I get the following error:

download.hs:24:1: error:
    Could not find module ‘Network.HTTP’
    Perhaps you meant Network.TLS (needs flag -package-key tls-1.5.2)
   |
24 | import "HTTP" Network.HTTP
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^

My GHC version is:

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.6.5

I also get errors like:

download.hs:22:1: error:
    Could not load module ‘Control.Concurrent.Async’
    It is a member of the hidden package ‘async-2.2.2’.
    You can run ‘:set -package async’ to expose it.
    (Note: this unloads all the modules in the current scope.)
   |
22 | import "async" Control.Concurrent.Async (mapConcurrently)
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I think it's possible there have been breaking changes between the ghc versions, and the examples I am finding online to start with may be outdated.

Any pointers on doing started with Haskell, and particularly easy ways to download and diff JSON files in Haskell?

I have been following this example: Running parallel URL downloads in Haskell, this is where I got the code that is erroring now.

Improve this question
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  • 2
    You appear to simply be missing the packages that define those modules; they need to be installed. Commented Nov 7, 2019 at 17:43
  • 2
    You almost certainly don't need the package name in the import statement: "You probably don’t need to use this feature, it was added mainly so that we can build backwards-compatible versions of packages when APIs change." Commented Nov 7, 2019 at 17:47
  • @chepner Do you mean the "HTTP" isn't necessary in import "HTTP" Network.HTTP, that it could just be import Network.HTTP? Also, how do I install these packages? I know you use cabal, but how do you know which package is missing? Should I try cabal install --lib http? Commented Nov 7, 2019 at 17:54
  • 1
    Unless you have Network.URL in both the HTTP and url packages, GHC is smart enough to find the correct package. Commented Nov 7, 2019 at 17:56
  • I am trying cabal install --lib network now ... I removed "HTTP" and other package names from the imports. Commented Nov 7, 2019 at 17:58
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