CSE341: Programming Languages Lecture 23 OO vs. Functional Decomposition; Adding Operations & Variants; Double-Dispatch Dan Grossman Fall 2011 Breaking things down • In functional (and procedural) programming, break programs down into functions that perform some operation • In object-oriented programming, break programs down into classes that give behavior to some kind of data This lecture: – These two forms of decomposition are so exactly opposite that they are two ways of looking at the same “matrix” – Which form is “better” is somewhat personal taste, but also depends on how you expect to change/extend software – For some operations over two (multiple) arguments, functions and pattern-matching are straightforward, but with OOP we can do it with double dispatch (multiple dispatch) Fall 2011 2 CSE341: Programming Languages The expression example Well-known and compelling example of a common pattern: – Expressions for a small language – Different variants of expressions: ints, additions, negations, … – Different operations to perform: eval, toString, hasZero, … Leads to a matrix (2D-grid) of variants and operations – Implementation will involve deciding what “should happen” for each entry in the grid regardless of the PL Fall 2011 3 CSE341: Programming Languages eval toString hasZero … Int Add Negate … Standard approach in ML • Define a datatype, with one constructor for each variant – (No need to indicate datatypes if dynamically typed) • Define a function for each operation • So “fill out the grid” via one function per column with one case-expression branch for each grid position – Can use a wildcard pattern if there is a default for multiple entries in a column See lec23_stage1.sml Fall 2011 4 CSE341: Programming Languages eval toString hasZero … Int Add Negate … Standard approach in OOP • Define a class, with one abstract method for each operation – (No need to indicate abstract methods if dynamically typed) • Define a subclass for each variant • So “fill out the grid” via one class per row with one method implementation for each grid position – Can use a method in the superclass if there is a default for multiple entries in a column See lec23_stage1.rb and lec23_stage1.java Fall 2011 5 CSE341: Programming Languages eval toString hasZero … Int Add Negate … A big CSE341 punchline • FP and OOP often doing the same thing in exact opposite way – Organize the program “by rows” or “by columns” • Which is “most natural” may depend on what you are doing (e.g., an interpreter vs. a GUI) or personal taste • Code layout is important, but there’s no perfect way since software has many dimensions of structure – Tools, IDEs can help with multiple “views” (e.g., rows / columns) Fall 2011 6 CSE341: Programming Languages eval toString hasZero … Int Add Negate …Now for stage 2: FP • For implementing our grid so far, SML / Racket style usually by column and Ruby / Java style usually by row • But beyond just style, this decision affects what (unexpected?) software extensions are easy and/or do not change old code • Functions: – Easy to add a new operation, e.g., noNegConstants – Adding a new variant, e.g., Mult requires modifying old functions, but ML type-checker gives a to-do list if we avoided wildcard patterns in Stage 1 Fall 2011 7 CSE341: Programming Languages eval toString hasZero noNegConstants Int Add Negate Mult Now for stage 2: OOP • For implementing our grid so far, SML / Racket style usually by column and Ruby / Java style usually by row • But beyond just style, this decision affects what (unexpected?) software extensions are easy and/or do not change old code • Objects: – Easy to add a new variant, e.g., Mult – Adding a new operation, e.g., noNegConstants requires modifying old classes, but Java type-checker gives a to-do list if we avoided default methods in Stage 1 Fall 2011 8 CSE341: Programming Languages eval toString hasZero noNegConstants Int Add Negate Mult The other way is possible • Functions allow new operations and objects allow new variants without modifying existing code even if they didn’t plan for it – The programming style “just works that way” • Functions can support new variants somewhat awkwardly “if they plan ahead” – See datatype 'a ext_exp and eval_ext at bottom of lec23.sml if interested • Objects can support new operations somewhat awkwardly “if they plan ahead” – The popular Visitor Pattern (not shown here), which uses the double-dispatch pattern (used next for another purpose) Fall 2011 9 CSE341: Programming Languages Thoughts on Extensibility • Making software extensible is valuable and hard – If you know you want new operations, use FP – If you know you want new variants, use OOP – If both? Languages like Scala try; it’s a hard problem – Reality: The future is often hard to predict! • Extensibility is a double-edged sword – Code more reusable without being changed later – But makes original code more difficult to reason about locally or change later (could break extensions) – Often language mechanisms to make code less extensible (ML modules hide datatypes; Java’s final prevents subclassing/overriding) Fall 2011 10 CSE341: Programming Languages Stage 3: Binary operations • Situation is more complicated if an operation is defined over multiple arguments that can have different variants – Can arise in original program or after an extension • Our example: – Include variants String and Rational – (Re)define Add to work on any pair of Int, String, Rational in either order • String-concatenation if >= 1 arg is a String, else math – (Just to keep example smaller, Negate and Mult still work only on Int, with a run-time error for a String or Rational) Fall 2011 11 CSE341: Programming Languages Binary operation in SML Add works differently for most combinations of Int, String, Rational – Run-time error for any other kinds of expression Natural approach: pattern-match on the pair of values – For commutative possibilities, can re-call with (v2,v1) Fall 2011 12 CSE341: Programming Languages fun add_values (v1,v2) = case (v1,v2) of (Int i, Int j) => Int (i+j) | (Int i, String s) => String (Int.toString i ^ s) | (Int i, Rational(j,k)) => Rational (i*k+j,k) | (Rational _, Int _) => add_values (v2,v1) | … (* 5 more cases
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