[Hugs-bugs] coredump on innocent code

[Ross Paterson]

On Sat, Sep 10, 2005 at 03:18:16PM -0000, fool at sdf-eu.org wrote:
> I previously thought that rewriting
> 
>   let <a> = <b>
>       <c> = <d> in <e>
> 
> as
> 
>   let (<a>,<c>) = (<b>,<d>) in <e>
> 
> is harmless, but section 3.12 of the Report says different.

Yes, you'd have to write it as

    let (~<a>,~<c>) = (<b>,<d>) in <e>

> I still don't understand why
> 
>   main = print (let (x,(q,_)) = (1,divMod x x) in q)
> 
> and
> 
>   main = print (let (x,(q,_)) = (1,divmod x x) in q)
>   divmod x y = (div x y,mod x y)
> 
> behave differently.

In both cases, to for x is to ask for the rhs to be reduced to the form
(<a>,(<b>,<c>)).  In the former case this can't be done without knowing
the value of x; in the latter it can.