apparently no way to implement --foo[=val]

It's a fairly common pattern for an option --foo to set some default value, and --foo=val to set some more specific value. (With the equals sign being required before the value; a space is ambiguous in this case.)

I cannot find a way to implement this with optparse-applicative, without renaming one of the options from --foo to --bar.

This code compiles, and usage shows "[--foo ARG] | [--foo]", but parsing "--foo" fails.

data Foo = FooStr String | FooBool Bool

parser = (FooStr <$> strOption (long "foo")) <|> (FooBool <$> switch (long "foo"))

I don't really like that as a way of implementing it even if it worked, due to the redundancy.

Could there be a version of option that takes a default value, for when no value is provided?

(#242 touches on this, but is also about a regression)

Start with the option and switch parser combinators described in the issue, including the strOption, switch, and option entry points. Determine how one --foo spelling could distinguish a bare flag from a required --foo=val, then verify that the parser accepts both forms without requiring duplicate option declarations.