CS G262 Assignment #1.
Assigned:  Thursday, 9 September 2004
Due:       Thursday, 16 September 2004, by 6:30pm

Implement the Qtype and Tenv ADTs that are specified below.
Your code must be written in portable (R5RS) Scheme.

The instructor is providing temporary implementations of the
VOID variable and of the Qsymbol ADT that are mentioned by
the specification of the Qtype and Tenv ADTs.  This code is
in /course/csg262/Assignments/Assignment1/qsymbol.sch.  Your
code should also work with any other correct implementation
of VOID and Qsymbol.

Submit this assignment as a printed listing of your commented
code.  Please print the listing using a fixed width font for
Scheme code.

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Specification of the Qtype and Tenv ADTs.

Both Qtype and Tenv are immutable ADTs.

Signature:

  var:              T                              -> Qtype
  type:             T                              -> Qtype
  isVar?:           Qtype                          -> boolean
  isType?:          Qtype                          -> boolean
  untagged:         Qtype                          -> T

  empty:                                           -> Tenv
  extend:           Tenv x Qsymbol x Qtype         -> Tenv
  lookup:           Tenv x Qsymbol                 -> Qtype
  
Algebraic specification:

    (isVar? (var t))  =  true
    (isVar? (type t)) =  false

    (isType? (var t))  =  false
    (isType? (type t)) =  true

    (untagged (var t))  =  t
    (untagged (type t)) =  t

    (lookup (empty) I)  =  type(VOID)

    (lookup (extend env I1 qt) I0)
        =  qt                          if (Qsymbol.equals I1 I0)

    (lookup (extend env I1 qt) I0)
        =  (lookup env I0)                             otherwise

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