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{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE ImportQualifiedPost #-}
{-# LANGUAGE TypeFamilies #-}
import Data.Kind (Constraint, Type)
import Data.Proxy (Proxy (Proxy))
import Data.Vector (Vector, cons, empty, ifilter, unsafeIndex, (//))
import Fcf qualified as F
import Fcf.Data.List qualified as F (Cons)
import Fcf.Data.Nat qualified as F (Nat)
import GHC.OverloadedLabels (IsLabel, fromLabel)
import GHC.TypeLits qualified as T
import Unsafe.Coerce (unsafeCoerce)
data Any (f :: k -> Type) where
Any :: f t -> Any f
data OpenProduct (f :: k -> Type) (ts :: [(T.Symbol, k)]) where
OpenProduct :: Vector (Any f) -> OpenProduct f ts
type UniqueKey (key :: k) (ts :: [(k, t)]) =
F.Null F.=<< F.Filter (F.TyEq key F.<=< F.Fst) ts
type family
RequireUniqueKey
(result :: Bool)
(key :: T.Symbol)
(t :: k)
(ts :: [(T.Symbol, k)])
:: Constraint
where
RequireUniqueKey 'True key t ts = ()
RequireUniqueKey 'False key t ts =
T.TypeError
( 'T.Text "Attempting to add a field named `"
'T.:<>: 'T.Text key
'T.:<>: 'T.Text "' with type "
'T.:<>: 'T.ShowType t
'T.:<>: 'T.Text " to an OpenProduct."
'T.:$$: 'T.Text "But the OpenProduct already has a field `"
'T.:<>: 'T.Text key
'T.:<>: 'T.Text "' with type "
'T.:<>: 'T.ShowType (F.Eval (LookupType key ts))
'T.:$$: 'T.Text "Consider using `update' instead of `insert'."
)
type FindIndex (key :: T.Symbol) (ts :: [(T.Symbol, k)]) =
F.FindIndex (F.TyEq key F.<=< F.Fst) ts
-- Exercise 12-i
-- Add helpful type errors to `OpenProduct`'s `update` and `delete` functions.
-- My take is a bit different than Sandy's solution. `FindElemWithBase`
-- generalizes element finding, so we can re-implement the original `FindElem`
-- (which gets stuck) plus a new `FindElemDuring` (which provides the helpful
-- error message) using it.
type FindElemWithBase (key :: T.Symbol) (ts :: [(T.Symbol, k)]) (base :: T.Nat) =
F.Eval (F.FromMaybe base F.=<< FindIndex key ts)
type FindElem (key :: T.Symbol) (ts :: [(T.Symbol, k)]) =
FindElemWithBase key ts F.Stuck
type FindElemDuring (action :: T.Symbol) (key :: T.Symbol) (ts :: [(T.Symbol, k)]) =
FindElemWithBase
key
ts
( T.TypeError
( 'T.Text "Attempting to "
'T.:<>: 'T.Text action
'T.:<>: 'T.Text " a field named `"
'T.:<>: 'T.Text key
'T.:<>: 'T.Text "' in an OpenProduct."
'T.:$$: 'T.Text "But the OpenProduct does not have a field `"
'T.:<>: 'T.Text key
'T.:<>: 'T.Text "'."
)
)
type FindableDuring action key ts =
( T.KnownNat (FindElemDuring action key ts)
, T.KnownNat (FindElem key ts)
)
type LookupType (key :: k) (ts :: [(k, t)]) =
F.FromMaybe F.Stuck F.=<< F.Lookup key ts
type UpdateElem (key :: T.Symbol) (t :: k) (ts :: [(T.Symbol, k)]) =
F.SetIndex (FindElem key ts) '(key, t) ts
type DeleteElem (key :: T.Symbol) (ts :: [(T.Symbol, k)]) =
F.Filter (F.Not F.<=< F.TyEq key F.<=< F.Fst) ts
type UpsertElem (key :: T.Symbol) (t :: k) (ts :: [(T.Symbol, k)]) =
F.UnMaybe
(F.Cons '(key, t) ts)
(F.ConstFn (F.Eval (UpdateElem key t ts)))
F.=<< FindIndex key ts
data Key (key :: T.Symbol) = Key
instance key ~ key' => IsLabel key (Key key') where
fromLabel = Key
class MaybeIndex (m :: Maybe F.Nat) where
maybeIndex :: Maybe Int
instance MaybeIndex 'Nothing where
maybeIndex = Nothing
instance T.KnownNat a => MaybeIndex ('Just a) where
maybeIndex = Just $ fromIntegral $ T.natVal $ Proxy @a
nil :: OpenProduct f '[]
nil = OpenProduct empty
insert
:: RequireUniqueKey (F.Eval (UniqueKey key ts)) key t ts
=> Key key
-> f t
-> OpenProduct f ts
-> OpenProduct f ('(key, t) ': ts)
insert _ ft (OpenProduct v) = OpenProduct $ cons (Any ft) v
findElem
:: forall key ts
. T.KnownNat (FindElem key ts)
=> Int
findElem = fromIntegral $ T.natVal $ Proxy @(FindElem key ts)
get
:: forall key ts f
. FindableDuring "get" key ts
=> Key key
-> OpenProduct f ts
-> f (F.Eval (LookupType key ts))
get _ (OpenProduct v) = unAny $ unsafeIndex v $ findElem @key @ts
where
unAny (Any a) = unsafeCoerce a
update
:: forall key ts t f
. FindableDuring "update" key ts
=> Key key
-> f t
-> OpenProduct f ts
-> OpenProduct f (F.Eval (UpdateElem key t ts))
update _ ft (OpenProduct v) = OpenProduct $ v // [(findElem @key @ts, Any ft)]
delete
:: forall key ts f
. FindableDuring "delete" key ts
=> Key key
-> OpenProduct f ts
-> OpenProduct f (F.Eval (DeleteElem key ts))
delete _ (OpenProduct v) = OpenProduct $ flip ifilter v $ curry $ (findElem @key @ts ==) . fst
upsert
:: forall key ts t f
. MaybeIndex (F.Eval (FindIndex key ts))
=> Key key
-> f t
-> OpenProduct f ts
-> OpenProduct f (F.Eval (UpsertElem key t ts))
upsert _ ft (OpenProduct v) =
case maybeIndex @(F.Eval (FindIndex key ts)) of
Nothing -> OpenProduct $ cons (Any ft) v
Just i -> OpenProduct $ v // [(i, Any ft)]
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