lenteja
what's this?
Suppose you have a strongly-typed Haskell data structure, and you want to explore and dig into it using lensy-like expressions entered interactively at runtime.
The program would have to parse the lensy-like expressions and assemble an optic from them, but how? What would be required? Ain't that too much dynamicity for Haskell?
This repo is a minimal proof-of-concept.
How does it work?
Basically, there's a HasLentejas typeclass. For types that are instances of this typeclass, a map of existentially quantified optics can be produced. The optics go from the type to each of its (existentially quantified) fields.
What could be improved?
Right now the HasLentejas instances are manually declared; perhaps they could be generically derived.
Support for setters and traversals could be added (working over a record value kept in a mutable reference).
Also Ix- and At- like functionality could be added, to allow indexing over list fields.
Right now the "end representation" uses Show, but perhaps it could be configurable and allow using something like ToJSON.
Support for listing available optics. Also, autocompletion.
example of use
Enter lensy exp:
name
A single result:
"John"
Enter lensy exp:
partner . name
A single result:
"Sara"
Enter lensy exp:
pets . folded
Multiple results:
Pet {petName = "Fido", petAge = 4}
Pet {petName = "Fifi", petAge = 3}
Enter lensy exp:
pets . folded . petName
Multiple results:
"Fido"
"Fifi"