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haskellghci

In ghci, how to remove an existing binding?

I am getting a "binding shadows the existing binding" error similar to the one from this question.

Prelude Api.Facility Control.Monad.IO.Class> let t = getBadgesNot 1 (Nothing) (Just 1)

<interactive>:55:5: warning: [-Wname-shadowing]
    This binding for ‘t’ shadows the existing binding
      defined at <interactive>:39:5

I defined the existing binding earlier in the session, and am now trying to redefine it. Is there a way to remove the existing binding so that I can redefine t?

I notice that in other circumstances ghci does not error when redefining an existing binding. For example

Prelude> let t = 1
Prelude> let t = 2
Prelude> let t = "there"

Why does ghci error when redefining an existing binding in some cases and not in others?

Solution

  • Is there a way to remove the existing binding so that I can redefine t?

    No, you cannot remove the existing binding. However, you can redefine t at any time, no problem.

    Why does ghci error when redefining an existing binding in some cases and not in others?

    Because you ran ghci with different warning/error settings; e.g. by passing -Wname-shadowing on the command line (perhaps because you ran ghci through cabal or stack, and the associated project specifies this option in its .cabal file). N.B. -Wname-shadowing should not prevent you from redefining t unless combined with -Werror to turn the mere warning into a full-blown error.

    The behavior also appears to differ depending on whether you use let or not; this is probably a bug:

    % ghci -Wname-shadowing -Werror
> let t=3
> let t=4
<interactive>:3:5: warning: [-Wname-shadowing]
    This binding for ‘t’ shadows the existing binding
      defined at <interactive>:1:5

<no location info>: error: 
Failing due to -Werror.
> t
3
> t=4
> t
4