Add fromAlt

[Fresheyeball]

I find this function useful.

[phadej]

This is good function to have. Few comments:

• Mention optional :: Alternative f => f a -> f (Maybe) in the haddock
• I'd like some different name (optional isn't fromAlt), but I don't know what it should be
• I'd like the x to be tried once; now

fromAlt launchMissles (throw "oops")

will launch missiles twice. (Or more practically, e.g. in parsers you'd confuse error reporting. You might get "expected x, or y, or x" diagnostic).

[Fresheyeball]

Mention optional :: Alternative f => f a -> f (Maybe) in the haddock

Can you explain why? I don't get it.

I'd like some different name (optional isn't fromAlt), but I don't know what it should be

What do you think about just alternative?

I'd like the x to be tried once; now

I don't understand this comment either.

[phadej]

• http://hackage.haskell.org/package/base-4.11.1.0/docs/Control-Applicative.html#v:optional
• I'd prefer the name which doesn't mention Alternative. There could be

-- or 'alternative'
fromAlt :: f a -> f b -> Either a b
fromAlt x y = Left <$> x <|> Right <$> y

I'd like a name referring to These. But as said, it's my problem, if you have no better suggestion.

Third point, I cannot explain better.

fromAlt launchMissles (throw "oops")
   = These <$> launchMissiles <*> throw "oops"
       <|> That <$> throw "oops"
       <|> This <$> launchMissiles
   = ...
   = launchMissiles *> (This <$> launchMissiles)
   /= This <$> launchMissiles

[Fresheyeball]

How about optionally? or maximize?

I still dont know what to day about optional in the haddocks, do you want to just edit the haddocks to say what you want?

I get your concern now about the order of operation. Will fix.