Is it possible to construct a new record using Lenses?

Question

If I have a record type with lenses, is it possible to construct a new record without using the underlying record accessors?

{-# LANGUAGE TemplateHaskell #-}

import Control.Lens
import Control.Lens.TH

data Foo = Foo { _s :: String
               , _b :: Bool
               } deriving (Show, Eq)

makeLenses ''Foo

I could make Foo an instance of Data.Default and then modifiy def with lenses, but not all record types will have sensible defaults. Does Control.Lens have its own way to do it?

Tanjatanjore

You could always use Foo{} as default, leaving all fields undefined. – Wisent

Thanks! I didn't even know you could omit fields there. I just assumed it would be a compile error. – Tanjatanjore

Note that this won't work if Foo has strict fields. – Magisterial

Good point @shachaf, I didn't consider that. So Default instance is required for strict fields and therefore probably is the correct way to go in general to avoid fragile code. – Tanjatanjore

Also note that you run into an issue with type-changing lenses. E.g. for data Foo a = Foo { _x, _y :: a }, the lenses x and y individually can't change the type, since you have to modify the fields x and y at once. – Magisterial

(There's a proposed mechanism that could be used to make type-changing lenses that would work there, but I don't think anyone's written code for it so far.) – Magisterial

So, in conclusion: This is a minefield! :) – Tanjatanjore

Answer 1

No, there is currently no way to do that. You'll have to use something like Foo{} as default or not use lens for record construction. However, there is already an issue in lens covering this.

Sapphism

Thanks for link to the github ticket. Some interesting discussion there! – Tanjatanjore