PLQ #3Done on:   Tuesday, February 8th

Question 1 @ 2022-02-08 18:34

Examine the eval function below, which operates directly on the given S-expression instead of parsing it into an AST first.

Which of the following statement(s) is true?

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• 1
  Separating semantics and syntax improves language extensibility.
• 2
  Separating semantics and syntax improves time/space complexity.
• 3
  Separating semantics and syntax improves ability to validate syntax.
• 4
  Separating semantics and syntax facilitates changes in syntax without affecting semantics.
• 5
  It is recommended that syntax and semantics be combined.

Question 2 @ 2022-02-08 18:36

We defined substitution using which of the following terms?

(Choose all of the relevant terms)

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• 1
  Compositionality
• 2
  Scope
• 3
  Bound occurrence
• 4
  Eager evaluation
• 5
  Lazy evaluation
• 6
  Binding instance
• 7
  Free occurrence
• 8
  Interpretation
• 9
  Compilation

Question 3 @ 2022-02-08 18:40

Which should be a correct way of computing substitution?

Starting from:

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• 1
  --> {+ {+ 5 5} {+ 5 5}} --> {+ 10 10} --> 20
• 2
  --> {with {x 10} {+ x x}} --> {+ 10 10} --> 20
• 3
  --> {with {x 20} {+ x x}} --> {+ 20 20} --> 40
• 4
  --> {with {x 10} {+ 10 10}} --> {+ 10 10} --> 20

Question 4 @ 2022-02-08 18:42

How would we interpret this: e[v/x]

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• 1
  Evaluate e where x is substituted by v
• 2
  Evaluate e where v is substituted with x
• 3
  Evaluate e where the given parameter is equal to dividing v by x
• 4
  This is an example for “M-expression” syntax, which we’re not using
• 5
  Evaluate v, replace the xs in e by the result, and continue

Question 5 @ 2022-02-08 18:43

What would this evaluate to?

If we were in our eager language, that would lead to a runtime error about y being unbound. But how would that change if we were using a lazy implementation (not evaluate the named expression)?

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• 1
  That would make it a syntax error
• 2
  It is impossible, since we cannot use y as a value
• 3
  We will still get the same runtime error (“unbound y”)
• 4
  Same error but about x (“unbound x”)
• 5
  We will get a 7
• 6
  We will get some undefined value result (void in Racket)