Type Propagation Example

Assume that the sum is the last component added; the following steps occur:

• When the summand is selected, the type integer becomes the accumulator type for the sum.

• The sum sends its accumulator type to find-update.

• find-update sends its accumulator type to alist.

• alist infers its element type (a combination of key and accumulator) and its data structure type (a list of elements) and sends its data structure type back to find-update.

• find-update sends the data structure type to iter-acc as its accumulator type.

• iter-acc will declare a variable to hold the alist.

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