input format

a problem we avoided

• all our interpreters so far have taken s-expressions as input
• parens gave nested list structure
• REP loop parses s-expressions into nested lists for us

example

• consider the expression
  ‘((y := 2) (x := (y + 1)) (print (x + y)))
  
• parsed by REP as a list of lists [draw BPD]
• corresponds to this abstract syntax tree

y

2

+

1

print

assign

x

assign

expr

y

+

y

x

program

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