[Haskell-cafe] Split the IsList class for OverloadedLists
lysxia at gmail.com
Fri Oct 30 13:48:10 UTC 2020
Right now the IsList class used by the OverloadedLists extension
requires both fromList, to construct something using list syntax, and
toList, to pattern-match on something using list syntax.
So types implementing that class are expected to be isomorphic to lists.
This is a very strong restriction. In particular, this gets in the way
of implementing IsList for aeson's Value type, since there's no
sensible total implementation of toList.
: That is a recently proposed idea
Proposal: split toList and fromList in two separate classes.
(And modify the OverloadedLists extension accordingly.)
Since they rely on an associated type family Item, it would be made a
type family Item (l :: Type) :: Type
class ToList l where
toList :: l -> [Item l]
class FromList l where
fromList :: [Item l] -> l
fromListN :: Int -> [Item l] -> l
(Note: we can't just replace ToList with Foldable, because these classes
have different kinds.)
- Any objections? An obvious concern is backwards compatibility. Is that
a deal breaker? Are there other issues with this idea?
- Should that be a GHC proposal?
- Has this been discussed before?
One alternative is to use RebindableSyntax, which already allows users
to redefine toList and fromList however they want. The issue is it might
also mess with all other kinds of syntactic sugar just enough that the
unpleasantness is not worth the trouble.
For example, if you wanted to use multiple list-like types in one
module, you would want an overloaded version of fromList/fromListN. You
either roll your own or find a suitable dependency. Either way it's
overhead you might not be willing to pay for, as opposed to something
that's already in base and Just Works.
So even with some existing workarounds in mind, the above proposal seems
a net improvement over the status quo.
Maybe some day we'll also get to take fromInteger out of Num.
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