(parse-1 st (? filename))
  st: a str
  filename: a str (optional)
  returns any type
F

Parses exactly one value from a string.

Equivalent to:

(match (parse-all st filename)
  ((val) val)
  (() (bail "parse-1 did not produce a form"))
  (_ (bail "parse-1 produced multiple forms")))
←parse-all
⭡
unparse→