Instaparse: there's an error but it is not reported -

i trying build grammar instaparse. quite find code fails first assertion, emitting "empty list":

(defn parse-it []   (let [parser (insta/parser ebnf)         res (insta/parses parser input)         _ (assert (seq res) (str "empty list"))         choices (count res)         _ (assert (= choices 1))]     (first res))) 

i resolve problem, involves trial , error. there way error can pinpointed?

an example of fixing problem removing trailing spaces file becomes input in above code.

edit

based on stefan's answer i've changed code:

(defn parse-it []   (let [my-parser (insta/parser ebnf)         xs (insta/parses my-parser input)         num-choices (count xs)         msg (cond               (zero? num-choices) "insta/parses might able corrected"               (> num-choices 1) (str "insta/parses shows more 1 way parse: " num-choices)               (= 1 num-choices) "insta/parses shows 1 choice, good")         res (cond               ;; fix there being more 1 rather show them               ;(> num-choices 1) xs               (= num-choices 1) (first xs)               (zero? num-choices) (insta/parse my-parser input))         _ (assert res (str "no result. num of choices is: " num-choices))]     [msg res])) 

the above code trick: pinpointed answer. me not obvious after insta/parses returns empty list, insta/parse needs called error information. using parse errors documentation result in better code above. shows how error information there in metadata, , how retrieve - answer question in documentation!

when use parser on input instead of going through insta/parses, prints pretty exact message of error @ repl.

an example:

(def ebnf   "expr = dot       = 'a'    dot  = '.'")  user> ((insta/parser ebnf) "a.") [:expr [:a "a"] [:dot "."]] user> ((insta/parser ebnf) "b.") parse error @ line 1, column 1: b. ^ expected: "a"