Is there a way to transform a pattern into a pattern of Booleans, with rests mapped to false, and anything else mapped to true? For example `"~ 2 ~ 2"`

becomes `"f t f t"`

.

I have tried this, which I thought could work for positive integers: `(> 0) <$> "~ 2 ~ 2"`

. However, while numbers are mapped true, rests seem to be mapped to nothing. And unfortunately, many functions like `while`

etc, require the false values to be there.

And a further question, can anyone point me to the documentation for `<$>`

? It is ungooglable!