Now that I have a version of ghc with type classes, I have had a go
at implementing records based on the ideas I mentioned on this list a
few months ago. The code of my first attempt is available at http://
homepage.ntlworld.com/b.hilken/files/Records.hs
I am releasing this to get feedback. I think Haskell needs a records
system of this kind of generality, and this code at least allows you
to play around.
From the comment section of the file:
-----------------------
Record construction:
EmptyRec is the empty record.
N =: x is the record with one field labelled N
carrying data x.
t +: u is the union of records t and u. Any overlap
of labels gives a static error.
Record destruction:
t .: N is the value of field N in record t. A lack
of field N gives a static error.
t -: N is record t with field N deleted. A lack of
field N gives a static error.
Record update:
t |: u is the record with fields from u where it
has them, t otherwise. If u has
any fields not in t, or of different types
from t, there is a static error.
Note that the result has the same type as t.
All these records have types:
EmptyRec is the type of the empty record.
N :=: a is the type of a record with one field
labelled N carrying data of type a.
r :+: s is the union of record types r and s. Any
overlap of labels gives a static error.
r :.: N is the type of field N in a record of type
r. A lack of field N gives a static error.
r :-: N is record type r with field N deleted. A
lack of field N gives a static error.
Finally some classes to govern the polymorphism:
r `Contains` N means that r is a record type with a
field labelled N.
r `Disjoint` s means that r and s are record types with
no fields in common.
r `Subrecord` s means that r and s are record types, and
every field of r also occurs in s (with the same type).
The types of the basic operators are as follows:
(=:) :: n -> a -> n :=: a
(+:) :: r `Disjoint` s => r -> s -> r :+: s
(.:) :: r `Contains` n => r -> n -> r :.: n
(-:) :: r `Contains` n => r -> n -> r :-: n
(|:) :: r `Subrecord` s => s -> r -> s
----------------------------------
Note that these records are a lot more expressive than the Hugs
system, as you can not only extend records by adding fields, but also
take unions of arbitrary (disjoint) records.
Record update is designed for functions with lots of named optional
arguments. If you define
f opts = ... options.:Optj ...
where
options = (Opt1 =: val1 +: ... +: Optn =: valn) |: opts
then the user can write (for example):
f (Optk =: u +: Optl =: v)
to set just two of the options, leaving the rest as default. This
also cannot be done in the Hugs system.
The main disadvantage of the current implementation is that you have
to tell the compiler in which order to store the fields, by defining
one of the following:
type instance NameCmp N M = NameLT
type instance NameCmp N N = NameEQ
type instance NameCmp N M = NameGT
for each pair of labels N & M in such a way as to give a linear order
on labels. You need n^2 definitions, where n is the number of labels.
I would do this in Template Haskell, but it won't yet allow you to
declare type instances. Maybe some compiler support?
Error messages tend to be cryptic. They mostly complain of missing
instances, and can run to several pages. There is really no way to
improve this without building it all in to the compiler!
All comments gratefully received, including suggestions on syntax,
choice of operators, implementation, explanation, etc.
Barney.
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