Convert a number to text
I added the following test code to investigate. It does an exhaustive check from 1 to 999 and then a few more cases.

import Data.Monoid
import Control.Monad
import Test.Hspec

specTest n w = it ("converts " &lt;&gt; show n) $ convert n `shouldBe` w

oneToNine = words "One Two Three Four Five Six Seven Eight Nine"
zeroToNineEndings = "" : map ("-"&lt;&gt;) oneToNine
oneToNinetyNine = oneToNine
    &lt;&gt; words "Ten Eleven Twelve Thirteen Fourteen Fifteen Sixteen Seventeen Eighteen Nineteen"
    &lt;&gt; [decade &lt;&gt; ending | decade &lt;- words "Twenty Thirty Forty Fifty Sixty Seventy Eighty Ninety", ending &lt;- zeroToNineEndings]

main = hspec $ do
  zipWithM_ specTest [1..]
    $ oneToNinetyNine
    &lt;&gt; mconcat
      [[d &lt;&gt; " Hundred"] &lt;&gt; map ((d &lt;&gt; " Hundred ") &lt;&gt;) oneToNinetyNine
      | d &lt;- oneToNine
      ]

  specTest 1000 "One Thousand and "
  specTest 1001 "One Thousand and One"
  specTest 1234 "One Thousand and Two Hundred Thirty-Four"
  specTest 1234567 "One Million, Two Hundred Thirty-Four Thousand and Five Hundred Sixty-Seven"


Note that 1000 converts to 'One Thousand and ' which is probably wrong. Also, your original question claims 1234 converts to 'One Thousand, Two Hundred Thirty Four' but in fact it comes out as 'One Thousand and Two Hundred Thirty-Four'.

I am not sure your rules for adding 'and' and commas in are correct, however I'm from the UK where we typically put an 'and' between the hundreds and tens (e.g. 'two hundred and thirty-four') and I'm aware that this isn't how things are done in the US. I'd recommend adding some more test cases as I've shown above to make sure you're getting things as you expect.

The code seems idiomatically ok, although the mixture of CamelCase and use_of_underscores is a little jarring. hlint offers some good advice on the use of idiom and reports this, a few unnecessary parentheses and $s and identifies that concat . intersperse can be written as intercalate. Nothing major.