Syntax error in ocaml -

why following code has syntax error? did not find out why.

let rec revstr stringa k e =  if    k = e "" else (string_of_char stringa.[e])^       (revstr stringa (e-1) k);; 

string a, k, e input , should give in output sub.string integer e integer k.

there no syntax error in code. when try "unbound value: string_of_char" type error. type errors appear after syntax errors, because typing occurs after parsing.

my guess have syntax error before function, such forgotten semi-colon or forgotten "in". kinds of errors detected later.