environments

what does Scheme have that our arithmetic expression language doesn’t?

• procedures
• variables and environments

these are interwoven because

• environments are needed to execute procedures
• basic idea of parameter binding:
  evaluate the body
  
  in an environment that has the parameters bound to the argument values

lexical scoping

• recall that in Scheme
  a procedure carries with itthe environment in which it was created
  
  and the environment in which its body is evaluated is an extension of this environment
• this is not so essential (but it’s very nice)

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