When you map across a string, it coerces the whole result into a new string character-by-character. It's like if you (hypothetically) weren't allowed to have nested lists but tried to do
(map [list 1 2 3 _] '(8 6 7 5 3 0 9))
Each string element is a character, not a whole string. Thus
arc> (= s (newstring 3))
"\u0000\u0000\u0000" ; these are null characters, by the way
arc> (= (s 0) #\a)
#\a
arc> s
"a\u0000\u0000"
arc> (= (s 1) #\b)
#\b
arc> s
"ab\u0000"
arc> (= (s 2) "foo")
Error: "string-set!: expects type <character> as 3rd argument, given: \"foo\"; other arguments were: \"ab\\u0000\" 2"
