failwiths ?here message value sexp_of_value raises an exception with the supplied
message and value, by constructing an Error.t and using Error.raise. As
usual, the sexp_of_value is only applied when the value is converted to a sexp or a
string. So, if you mutate value in between the time you call failwiths and the
time the error is displayed, those mutations will be reflected in the error message.
failwiths ?here s a f = Error.raise (Error.create ?here s a f)
When compiling with pa_fail (the default at jane street) the ?here argument is
automatically filled in with the value _here_, and so cannot be passed manually.
We say ?here:Lexing.position rather than ?here:Source_code_position.t to avoid a
circular dependency.
failwithp here is like failwiths ~here, except that you can provide a source
position yourself (which is only interesting if you don't provide _here_).