Please, inspect if demand & force is neede before {to,from}Value.

[Anton-Latukha]

Unless I am deepky mistaken, demanding and forcing before calling fromValue is redundant. The pattern is to use fromValue alone. Not sure about coerce without looking at the code, but I see no reason for it to be different.

Originally posted by @layus in #948 (comment)

[Anton-Latukha]

To be honest - I am still lost in the type class jumps inside the forest of Convert module, it is hard to say if all of them lead to demand/force.

HLS jumps to type class definition & does not support jumping to instance implementation, there is no tool that show those instance connections.

---

Seems like demand is not always called during conversion:

instance Convertible e t f m
  => FromValue Float m (NValue' t f m (NValue t f m)) where

  fromValueMay =
    pure .
      \case
        NVConstant' (NFloat b) -> pure b
        NVConstant' (NInt   i) -> pure $ fromInteger i
        _                      -> Nothing

  fromValue = fromMayToValue TFloat

fromMayToValue
  :: forall t f m a e
  . ( Convertible e t f m
    , FromValue a m (NValue' t f m (NValue t f m))
    )
  => ValueType
  -> NValue' t f m (NValue t f m)
  -> m a
fromMayToValue t v =
  maybe
    (throwError $ Expectation @t @f @m t (Free v))
    pure
    =<< fromValueMay v

That Float instance fromValue jumps into fromMayToValue which jumps back to Float fromValueMay.

---

I'd reformulated the question: where demand/force should be used & if they required - they should be included into according instances.

[soulomoon]

From my understanding:

Since MonadValue (StdValue m) m => type StdValue m = NValue (StdThunk m) (StdCited m) m

demand :: v -> m v
v = NValue t f m
m = NValue' t f m.

For fromValue/fromMayToValue

• For case v we have FromValue a m (NValue t f m) and FromValue a m (Deeper (NValue t f m)) instances where demand/force would be called.
• For case m v no demand is called.

(NValue t f m) would always be demand/force if chains of fromValue/fromMayToValue is used for every encounter.