Unexpected NoImplicitPrelude behaviour in GHCi (bug?)

[Philip K.F. Hölzenspies]

Dear GHCers,

With regards to the e-mail below, I have done one more test and found
that


ghci -fno-implicit-prelude


and


ghci -XNoImplicitPrelude


behave the same. However, I'm still a little taken aback by the use
thereof. Consider two files Foo.hs and Bar.hs:


Foo.hs:
{-# LANGUAGE NoImplicitPrelude #-}
module Foo where
import Bar
fromInteger  _ = ()
fromRational _ = ()
_ >> a = bar a

Bar.hs:
module Bar where
bar = id


If I load Foo into GHCi, I arrive at a prompt with a subset of the
Prelude in the context. Consider the following:

*Foo> :t map

<interactive>:1:0: Not in scope: `map'
*Foo> :t 5
5 :: (GHC.Num.Num t) => t
*Foo> 5
5
*Foo> 


If I load Foo into GHCi with an explicit command-line option
-XNoImplicitPrelude, module Bar fails to compile, because 'id' is
unknown. If I add an explicit import of the Prelude to Bar, running ghci
with the command-line option results in:

*Foo> :t map

<interactive>:1:0: Not in scope: `map'
*Foo> :t 5
5 :: ()
*Foo> 5
()
*Foo> 


Shouldn't the expected behaviour of GHCi be that the "entry module"
determines the entire context? In other words, if module X in


ghci X


or in


ghci
> :l X


contains the LANGUAGE-pragma NoImplicitPrelude, should the Prelude not
be unloaded from ghci?

I would argue that this might also be seen as an example of why Ticket
#124 for haskell-prime is a good idea for GHC:

http://hackage.haskell.org/trac/haskell-prime/ticket/124

Kind regards,
Philip


On Thu, 2010-06-10 at 11:59 +0200, Philip K.F. Hölzenspies wrote:
> Dear GHCers,
> 
> I have been experimenting some more with environments for lab work for
> an FP intro course. One thing students tend to have difficulty with in
> the initial labs are the error messages including type classes, or any
> kind of more general type than they expected. I am trying to work around
> this, by supplying a "Number" type for the first lab and gradually
> increasing the complexity over the next few labs. To let all error
> messages be in terms of my type, I use the NoImplicitPrelude option in a
> LANGUAGE pragma. However, I find the behaviour of GHCi unexpected. I
> have reduced the problem to a small case that reproduces the bug. Here
> is my BugDemo module:
> 
> {-# LANGUAGE NoImplicitPrelude #-}
> module BugDemo where
> import qualified Prelude as P
> newtype Number = N P.Integer
> fromInteger = N
> 
> Look at the following sessions:
> 
> 
> [holzensp at ewi1043:work/FPPrac]% ghci BugDemo.hs
> GHCi, version 6.12.1: http://www.haskell.org/ghc/  :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer-gmp ... linking ... done.
> Loading package base ... linking ... done.
> Loading package ffi-1.0 ... linking ... done.
> [1 of 1] Compiling BugDemo          ( BugDemo.hs, interpreted )
> Ok, modules loaded: BugDemo.
> *BugDemo> 5
> 5
> *BugDemo> :t 5
> 5 :: (P.Num t) => t
> *BugDemo> :q
> Leaving GHCi.
> [holzensp at ewi1043:work/FPPrac]% ghci -fno-implicit-prelude BugDemo.hs
> GHCi, version 6.12.1: http://www.haskell.org/ghc/  :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer-gmp ... linking ... done.
> Loading package base ... linking ... done.
> Loading package ffi-1.0 ... linking ... done.
> [1 of 1] Compiling BugDemo          ( BugDemo.hs, interpreted )
> 
> on the commandline:
>     Warning: -fno-implicit-prelude is deprecated: use
> -XNoImplicitPrelude or pragma {-# LANGUAGE NoImplicitPrelude #-} instead
> Ok, modules loaded: BugDemo.
> *BugDemo> 5
> 
> <interactive>:1:0: Not in scope: `>>'
> *BugDemo> :t 5
> 5 :: Number
> *BugDemo> 
> 
> 
> It's a bit awkward that I get an explicit warning about
> -fno-implicit-prelude being deprecated, while the behaviour is actually
> different. Obviously, when leaving out the definition for fromInteger,
> the second session fails whenever I type a number, whereas the first
> session behaves the same way.
> 
> This seems to be me to be a bug.
> 
> Kind regards,
> Philip
> 
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