This question uses query parameters in http-conduits as described in this previous SO answer.
We will use the method described here on SO to download the markup content of a page.
Although this task could be possible using the mediawiki, it seems significantly simpler to use the ?action=raw method without explicitly using the API.
In order to support different pages (e.g. en.wikimedia.org), I wrote two functions getWikipediaPageMarkup and getEnwikiPageMarkup, the former one being more general and allowing to use custom domains (any domain should work, assuming Mediawiki is installed under /wiki).
{-# LANGUAGE OverloadedStrings #-}
import Network.HTTP.Conduit
import Data.ByteString (ByteString)
import qualified Data.ByteString.Char8 as B
import qualified Data.ByteString.Lazy.Char8 as LB
import Network.HTTP.Types (urlEncode)
import Data.Monoid ((<>))
-- | Get the Mediawiki marup
getWikipediaPageMarkup :: ByteString -- ^ The wikipedia domain, e.g. "en.wikipedia.org"
                       -> ByteString -- ^ The wikipedia page title to download
                       -> IO LB.ByteString -- ^ The wikipedia page markup
getWikipediaPageMarkup domain page = do
    let url = "https://" <> domain <> "/wiki/" <> urlEncode True page
    request <- parseUrl $ B.unpack url
    let request' = setQueryString [("action", Just "raw")] request
    fmap responseBody $ withManager $ httpLbs request'
-- | Like @getWikipediaPageMarkup@, but hardcoded to 'en.wikipedia.org'
getEnwikiPageMarkup :: ByteString -> IO LB.ByteString
getEnwikiPageMarkup = getWikipediaPageMarkup "en.wikipedia.org"
Note that a recent http-conduit version is required (minimum: 2.1, tested with 2.1.4) in order to compile the code.