Records in Haskell

AntC anthony_clayden at
Tue Feb 28 12:40:26 CET 2012

Oliver Batchelor <saulzar <at>> writes:

> Hi,
> >
> > Wrong: You cannot use a fieldLabel `name` declared in module/namespace A to
> > access a record with a field `name` declared in module B. You'll get a 'no
> > instance' compile fail. Same familiar rules as for any instance resolution.
> >
> > This is the crucial difference compared to SORF: which can't control the 
> > of its String Kind. (Apologies that I added a speculative discussion of
> > whether DORF could use String Kinds. I said that if doing so would open
> > the 'back door' to the abstraction, then I'll stick with types.)
> >
> Perhaps this situation could occur though?
> Module A
> fieldLabel name String
> Module B
> import A -- unknowingly picking up the name label
> data Foo = Foo { name :: String } -- uses the name label by accident
> So there'd have to be some syntax to make sure you intend to use a
> label rather than accidentally use it?
> (Not that this is a big issue, the situation is surely minor compared
> to sharing unrelated labels all the time)
> Oliver

Thanks Oliver, hmm ...

Did module B import A unqualified?

Did module B not have its own declaration of fieldLabel `name`?
And presumably module B has set the option to use DORF.

Then DORF is going to take it that you mean to share the `name`. (And actually 
I don't see much harm resulting.)

Them's the rules.

If there's a fieldLabel `name` in Module B, Foo will use that.
If furthermore B imports A unqualified, that's a clash of fieldLabels, so 
compile fail.
If module B is compiled with H98 style records, there's a name clash with the 
H98 field selector function.

I think this is no worse (and no better) than business-as-usual 'accidental' 
usage-without-declaration matching an unknowingly imported binding.

(As part of the 'minimal changes' requirement, I'm trying to avoid syntax 
changes to record decls.)


More information about the Glasgow-haskell-users mailing list