It's not clear to me why it's necessary to include <T, U> at the start of the function definition. In Haskell, unquantified type variables are implicitly universally quantified, i.e. you assume that the signature must be valid for any types T and U.

That's the only thing I think could be substantially improved, though. Personally I find the style where you separate the type signature from the function definition easier to read, for example

How about

    <^> : (T -> U) * T? -> U?
    <^> (f, a) {
        return a.map(f)
    }

or even

    <^> : (T -> U) * T? -> U?
    <^> (f, a) -> a.map(f)

which is already pretty close to the Haskell equivalent

    (<^>) :: (t -> u) -> Maybe t -> Maybe u
    (<^>) = fmap

Arguably, Haskell syntax could be improved with more built-in syntax

    <^> : (t -> u) -> t? -> u?
    <^> = fmap

though I haven't thought about what this means for parsing the language.