Vinyl is a general solution to the records problem in Haskell using type level strings and other modern GHC features, featuring static structural typing (with a subtyping relation), and automatic row-polymorphic lenses. All this is possible without Template Haskell.
First, install Vinyl from Hackage:
cabal update
cabal install vinyl
To learn more, try this tutorial.
Vinyl is an approach to records in Haskell based on a heterogeneous list type: a list where each element may have a different type. Since it is a list, accessing any individual field of a Vinyl record seems as though it should have O(n) complexity, with n the size of the record. Sure enough, pattern matching on a Vinyl record works just as with a list: we peel off the head element until we reach the one we are looking for. But we have an option to change these asymptotics.
If we anticipate frequent get operations on a record, we can use the function toARec :: Rec f ts -> ARec f ts to produce an array-backed Rec that offers constant-time field access. We can consider a benchmark of getting specific fields from different varieties of Vinyl records.
This benchmark suggests the trends we should expect
- Getting a field deeper in a Vinyl record takes more time than getting one earlier (i.e. it appears to the left, or under fewer
:&constructors) in the record - Getting a field of an
ARec(orAFieldRec, which is anARecwith named fields) array-backed record does not depend on where in the record that field lies
The x-axis of this graph should be labeled as nanoseconds.
