What's the deal with fruit juice?

Question

Fruit juice seems to be a pretty worthless item on itself:

• For its intended purpose, it conveys a very low amount of nutrition and also a very low nutrition per weight ratio (it is 0.5, 1 or 1.5 depending on BUC status). Any food or corpse has a better ratio.
• There are only two Alchemical recipes that require fruit juice.
  • The potion of see invisible can be made once, then blessed, to give the permanent intrinsic.
  • The potion of booze has a single desirable effect (confusion), and it offers the same nutrition values that fruit juice does.
    • Also, both booze and fruit juice don't break foodless conduct.
• Throwing it doesn't appear to have an effect.
• Diluting the juice into water seems to be the best use for it. However, it isn't the purpose of the potion.

Are there any other uses (preferably practical) for fruit juice?

Answer 1

Short answer: no, there are no other uses. (I generally dilute and then bless/curse it to add to my stockpile of aligned water.)

According to the Nethack Spoiler List, that list of uses is comprehensive, but we can check that by reading the source.

POT_FRUIT_JUICE is mentioned in shknam.c (shop initialization), zap.c (wands) and potion.c (potions). shknam.c is just as an entry in the item list for delis, and zap.c is for the cancellation of potions of sickness and see invisible into fruit juice, so the meat is in potion.c.

Of the ten mentions in potion.c...

• two for the "this tastes like" handling
• one commented out in the terrifying edifice that is potionhit(), and another in potionbreathe()
• six in potionmix(), covering
  • unihorn + sickess => juice
  • amethyst + booze => juice
  • gain {level,energy} + juice => see invisible
  • fruit + {speed,enlightenment} => booze
  • fruit + sickness => sickness

And that's everything; it has no other properties that are specific to fruit juice (as opposed to shared with all potions like being dilutable).

As for why, the revision history of Nethack might tell us -- if it were publically available. But as far as I can tell, it's not.

Answer 2

There doesn't seem to be any obvious in-game "purpose" for the potion of fruit juice — it's just a mostly useless item, like the wand of nothing, the flint stone or the worthless pieces of glass.

Indeed, a quick source search shows that the potion of fruit juice has existed since Hack 1.0, where it did absolutely nothing except for reducing your hunger by 20 points.

The main "meta-game" reason for the existence of the potion of fruit juice appears to be making potion identification slightly more difficult. In particular, the potions of see invisible and fruit juice are deliberately hard to distinguish, unless you happen to have something invisible around, and they're actually implemented by the same code. (Apparently, this is also a shout-out to the original Rogue, although I haven't managed to determine when the potion of fruit juice was introduced there and what it originally did.)

---

As noted, unless you happen to be playing a conduct that makes the tiny nutrition gain from drinking fruit juice actually worthwhile, the main in-game use for fruit juice is to dilute it into water, which can then be blessed to make holy water.

However, if you're into heavy alchemy, the "fruit juice + speed → booze" recipe, useless as it seems on its own, turns out to be a crucial step in the complicated alchemy chain that allows you to turn almost any potions (including, yes, water) into a giant stack of potions of gain level, which you can then use to max your level and pump up your HP and energy even beyond that (or, if you prefer, curse them and use them to skip most of Gehennom on you way up after retrieving the amulet).

Ps. One very minor use that you didn't mention is that, if you happen to be short of cash, fruit juice can be sold to delicatessen shops in addition to potion shops and general stores. It's not worth much, but if you're just a few zorkmids short of a donation to the Minetown priest, and the general store vendor is out of cash, it could be useful.