Unexpected NoImplicitPrelude behaviour in GHCi (bug?)

"Philip K.F. Hölzenspies" p.k.f.holzenspies at utwente.nl
Tue Jun 15 07:19:23 EDT 2010

On Jun 11, 2010, at 5:10 AM, Brandon S. Allbery KF8NH wrote:


> This doesn't surprise me; when putting it in the module, it affects only that module.  When using either command line version, it affects *everything*... and what's breaking is not your definition of  Number, but the ghci expression printer (which, being in IO, is doing something like (print it >> putStr "\n").  Since the command line option has global effect, the Prelude's (>>) isn't defined for ghci's guts either.

Your explanation would make perfect sense to me; pragma's in the module effect only the module. This is, however, nog GHCi's behaviour. If I have a module that's only this:

{-# LANGUAGE NoImplicitPrelude #-}
module Foo where

and I open this in GHCi, I get the following session:

Ok, modules loaded: Foo.
*Foo> 42
*Foo> 42.0
*Foo> :t 42
42 :: (GHC.Num.Num t) => t
*Foo> 40 + 2

<interactive>:1:3: Not in scope: `+'
*Foo> :t fromRational

<interactive>:1:0: Not in scope: `fromRational'

This means that there is *some* definition of fromInteger, fromRational and (>>), but it's not accessible for me at the prompt. Types and typeclasses seem to be imported fully qualified. I would either expect GHCi to start complaining when I type 42, or to not complain about any of these above commands.

Any thoughts?


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