Functional programming paradigm common mistakes

Common mistakes

• Assuming every co-domain member must be an output

  Treating the co-domain as the exact set of values that the function produces.

  Fix itThe co-domain is the set from which outputs are chosen. It may contain members that the function never uses.

• Assuming first-class objects are only functions

  Treating the phrase first-class object as meaning that only functions can have first-class status.

  Fix itFunctions can be first-class objects, but the specification also gives integers, floating-point values, characters and strings as examples of first-class values in many programming languages.

• Treating the inputs as unrelated arguments

  Saying that a function with type integer x integer → integer takes two arguments in fact.

  Fix itAlthough it is commonly described as taking two arguments, it takes one argument: a pair of integers such as (3,4).

• Treating a partially applied function as the final result

  Assuming that add 4 is an integer result.

  Fix itRecognise that add 4 is a function of type integer → integer. It needs another integer before the final integer result is produced.

• Applying the functions in the wrong order

  Treating g ○ f as though g is applied first and f is applied second.

  Fix itApply the function nearest the input first. For g ○ f, apply f first, then apply g to the result.