scheme - define-type returning error on DrScheme -

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When I try to run a defined type on the dosim, I get an error for an identifier "before its definition" Is: defined-type ". Why is this happening?

I input: 

  (defined type GUI [label (text string?)] [Button (text string?) (Enabled? Boolean?)]

and returns to me:

Before its definition, reference to an identifier: defined-type

Defined-type is not part of the "basic" plan, it is a part of "typed scheme." To use this language in the DR scheme To:

Ensure that "modi In the bottom window, enter: 

  #lang type-scheme

and "run" in the bottom left of the window. "Press the button.


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