fromJust Nothing problem

Serge D. Mechveliani mechvel at
Wed Nov 15 03:09:02 EST 2006

On Tue, Nov 14, 2006 at 05:28:22PM +0000, Simon Peyton-Jones wrote:

> Both of these messages (from Neil and Serge) suggest use debugging ideas. 
> Would anyone like to add them to
> [..]

How can I put it there? It needs login.

I do not know, whether my idea worths it, but, for any occasion, here is 
its new formulation

Maybe, the simplest way to provide locating in the source code a mismatch
run-time error in the library functions
                                        head, tail, fromJust, and such,

is to  avoid  these functions and to use explicit matching instead.

For example, for the program     g x = h $ fromJust $ f x,

ghc-6.6  often looses the reference to  g, f, and h  in its run-time error
report -- when  f  returns  Nothing.
But for the program
                     g x =  let  Just y = f x  in  h y,

the ghc run-time report is like this:

    Main: M1.hs:9:11-22:
    Irrefutable pattern failed for pattern Data.Maybe.Just y

-- it points to the source line!

Serge Mechveliani
mechvel at

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