signature IO =
sig
type ioerr
exception Io of ioerr
val ioerrToString : ioerr -> string
end
structure IO : IO
type ioerr
The ioerr type is abstract, because we cannot reconcile the
different data carried by I/O errors in the SML and OCaml bases.
Consequently, the programmer must match Io’s argument using a
wildcard.