Thus quoth Tyson Whitehead at 02:19 on Thu, Apr 20 2017:
Interesting observation about loading it from a file via typing it in.
Initially I suspected the culprit was the monomorphism restriction being turned off by default (:showi language in GHCi, (great thanks to Brandon Allbery who mentioned this command on another thread)), but I don't really see a difference when I turn it off.
My code is actually a test case for use with the doctest package that I was just running manually to work out. I'm not sure if it does the equivalent of loading it from a file or not.
Oh, I see.
Ultimately I wound up doing the binding via a case statement instead of let. It seems GHCi is okay with that.
That's kind of funny. Seems somewhat inconsistent to my eye. -- Sergiu