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The ordering of reductions
Given a complex expression, there are many different orderings of the applicable reductions
Using normal-order reduction, the first reduction to perform is the one at the outer level of the expression (the leftmost reduction first).
Using applicative-order reduction, the first reduction to perform is the inner leftmost reduction.
- Normal-order reduction represents evaluation by need
- Applicative-order reduction evaluates all constituent expressions, some of which are
unnecessary or perhaps even harmful.
- As such, there is often a need to control the
evaluation process with special forms that use non-standard evaluation strategies