CODEWITHSUNDEEP
haskell
Marilyn
Marilyn

Reputation: 13

Why is it possible to build a Haskell program with .hs files that fail to compile individually?

I am relatively new to Haskell and the compile/build process. I cloned a Haskell package from Github that builds using a cabal file. I can build the program no problem using stack build, stack install etc.

Currently I am interested in using the functions in only the Rewrite.hs file in the Haskell package, but I cannot successfully compile it by importing Rewrite to my own Main.hs and calling ghc Main.hs. I get the following type errors:

Rewrite.hs:73:13: error:
    • Expecting one more argument to ‘Module’
      Expected a type, but ‘Module’ has kind ‘* -> *’
    • In the type signature:
        stripTop :: Module -> Module

Rewrite.hs:16:29: error:
    • Expecting one more argument to ‘Pat’
      Expected a type, but ‘Pat’ has kind ‘* -> *’
    • In the type signature:
        findPats :: Data a => a -> [Pat]

I thought that building a package basically compiles and links files together - Why is it possible to have compiler errors for parts of a program when compiling by hand? Alternatively, should I not be attempting to use a .hs file from a part of a program like this?

EDITED:

The package is https://github.com/Genetic-Strictness/Autobahn

An example of Main:

import Rewrite
import System.FilePath
import Language.Haskell.Exts

file ::FilePath
file = "test.hs"

main = placesToStrict file

where the function placesToStrict is from Rewrite which calls on findPats, which causes the compiler errors above.

Upvotes: 0

Views: 141

Answers (1)

homura
homura

Reputation: 231

The .cabal file specifies several language extensions: other-extensions: BangPatterns, FlexibleInstances, MultiParamTypeClasses, TypeSynonymInstances

You can enabled these on a per file basis by putting: {-# LANGUAGE BangPatterns, FlexibleInstances, ... #-} at the top of your source file. You can also pass -XBangPatterns -XFlexibleInstances ... to ghc, although this is discouraged.

Upvotes: 1

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