18.5.3 External formats and stream-element-type

The :element-type argument in open and with-open-file defaults to the value of lw:*default-character-element-type* .

If :element-type is not :default , checks are made to ensure that the resulting stream's stream-element-type is compatible with its external format:

1. If the direction is :input or :io , the argument must be a supertype of the type of characters produced by the external format.
2. If the direction is :output or :io , the argument must be a subtype of the type of characters accepted by the external format

If the element-type argument doesn't satisfy these requirements, an error is signalled.

If :element-type :default is passed, the system chooses the stream-element-type the basis of the external format.