[Haskell] Proposal: Allow "\=" for field update in record update syntax

Benjamin Franksen benjamin.franksen at bessy.de
Mon Feb 28 19:16:40 EST 2005

On Thursday 24 February 2005 23:27, Keean Schupke wrote:
> Benjamin Franksen wrote:
> >>Well at the moment this would give an error, but remember the
> >>list is heterogeneous, so you can just not give the list a type, and
> >>simply append the specific function... admitedly this is not as
> >>type-safe.
> >>
> >>hUpdateAtLabel field2 someFunction myRecord
> >
> >That is an advantage of hLists as compared to normal records.
> >
> >A disadvantage is that each field access needs to traverse the list. I
> > wonder if this isn't rather less efficient than the random access
> > provided by normal records.
> Well, not quite true, because the type of the label is used to index the
> value, the selection happens at compile time. So at run time there is no
> instance selection left... it is simply the value. At least in theory!
> whether
> the particular compiler/interpreter does this is implementation dependant.
> This is why we decided that the simpler to implement list was better than
> a more complex tree structure.

Hmm. I haven't seen it from this perspective, yet! At first reading, I thought 
this is simply too good to be true. I mean, there is some sort of list 
structured thing representing the whole record, right? Then how can the 
function that selects an element *not* traverse through the list?

After thinking for some time about this, my head begins to spin badly! I tend 
to believe now, that it could indeed be possible that the compiler performs 
the traversal at compile time, but the thought still gives me headaches.


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