Re: GHCi and line numbers (with ghc-7.4.1)

23 Mar 2012

      On 22/03/2012 11:36, Christopher Done wrote:
...
On 22 March 2012 12:13, Simon Marlow  wrote:
...
On 20/03/2012 20:12, Simon Hengel wrote:
...
They are now incremented with each evaluated expression.
Why *are* they incremented with each evaluation? Surely the only use
for line numbers would be in multi-line statements:
...
:{
Prelude| do x<- [1..10]
Prelude|    return y
Prelude| :}
<interactive>:6:11: Not in scope: `y'
Would it not make more sense to have
<interactive>:2:11: Not in scope: `y'
as it would do if compiling the file in a source file? From the older
GHCs, this always gives 1, indicating that multi-line statements are
somehow parsed and collapsed before being compiled, or maybe the line
number was just hard coded to 1.
One reason for having the line number is that it gets attached to 
declarations:

Prelude> let x = 3
Prelude> let y = 4
Prelude> :show bindings
x :: Integer = 3
y :: Integer = 4
Prelude> :i x
x :: Integer    -- Defined at <interactive>:20:5
Prelude> :i y
y :: Integer    -- Defined at <interactive>:21:5

I think another reason we added it was so that we could tell when a 
declaration has been shadowed:

Prelude> data T = A
Prelude> :i T
data T = A      -- Defined at <interactive>:25:6
Prelude> data T = B
Prelude> :i T
data T = B      -- Defined at <interactive>:27:6
Prelude>

admittedly it's not a super-useful feature, but if you're dealing with 
multiple bindings with the same name it does give you some confirmation 
that GHC is thinking about the same one that you are.

Cheers,
	Simon
...
FWIW, in my Emacs mode (making good progress on adding to
haskell-mode) I use the column number in the REPL to highlight on the
line where the problem is (e.g. here
http://chrisdone.com/images/hs-repl-error-demo.png), for GHC 7.* with
proper multi-line support I will automatically wrap any multi-line
expressions entered in the REPL in :{ and :}, it would be cool for
line numbers in errors to be useful for that. (Arguably we should be
using the GHC API and Scion or something like it, but these change
quite often and are hard to support whereas interfacing with GHCi is
quite stable across around seven releases and "just works".)
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