In the Eiffel user discussion group [1], Ian Joyner recently asked:
A lot of people are now using Result as a variable name for the return value in many languages. I believe this first came from Eiffel, but can’t find proof. Or was it adopted from an earlier language?
Proof I cannot offer, but certainly my recollection is that the mechanism was an original design and not based on any previous language. (Many of Eiffel’s mechanisms were inspired by other languages, which I have always acknowledged as precisely as I could, but this is not one of them. If there is any earlier language with this convention — in which case a reader will certainly tell me — I was and so far am not aware of it.)
The competing conventions are a return instruction, as in C and languages based on it (C++, Java, C#), and Fortran’s practice, also used in Pascal, of using the function name as a variable within the function body. Neither is satisfactory. The return instruction suffers from two deficiencies:
- It is an extreme form of goto, jumping out of a function from anywhere in its control structure. The rest of the language sticks to one-entry, one-exit structures, as I think all languages should.
- In most non-trivial cases the return value is not just a simple formula but has to be computed through some algorithm, requiring the declaration of a local variable just to denote that result. In every case the programmer must invent a name for that variable and, in a typed language, include a declaration. This is tedious and suggests that the language should take care of the declaration for the programmer.
The Fortran-Pascal convention does not combine well with recursion (which Fortran for a long time did not support). In the body of the function, an occurrence of the function’s name can denote the result, or it can denote a recursive call; conventions can be defined to remove the ambiguity, but they are messy, especially for a function without arguments: in function f, does the instruction
f := f + 1
add one to the value of the function’s result as computed so far, as it would if f were an ordinary variable, or to the result of calling f recursively?
Another problem with the Fortran-Pascal approach is that in the absence of a language-defined rule for variable initialization a function can return an undefined result, if some path has failed to initialize the corresponding variable.
The Eiffel design addresses these problems. It combines several ideas:
- No nesting of routines. This condition is essential because without it the name Result would be ambiguous. In all Algol- and Pascal-like languages it was considered really cool to be able to declare routines within routines, without limitation on the depth of recursion. I realized that in an object-oriented language such a mechanism was useless and in fact harmful: a class should be a collection of features — services offered to the rest of the world — and it would be confusing to define features within features. Simula 67 offered such a facility; I wrote an analysis of inter-module relations in Simula, including inheritance and all the mechanisms retained from Algol such as nesting (I am trying to find that document, and if I do I will post it in this blog); my conclusion was the result was too complicated and that the main culprit was nesting. Requiring classes to be flat structures was, in my opinion, one of the most effective design decisions for Eiffel.
- Language-defined initialization. Even a passing experience with C and C++ shows that uninitialized variables are one of the major sources of bugs. Eiffel introduced a systematic rule for all variables, including Result, and it is good to see that some subsequent languages such as Java have retained that convention. For a function result, it is common to ignore the default case, relying on the standard initialization, as in if “interesting case” then Result:= “interesting value” end without an else clause (I like this convention, but some people prefer to make all cases explicit).
- One-entry, one-exit blocks; no goto in overt or covert form (break, continue etc.).
- Design by Contract mechanisms: postconditions usually need to refer to the result computed by a function.
The convention is then simple: in any function, you can use a language-defined local variable Result for you, of the type that you declared for the function result; you can use it as a normal variable, and the result returned by any particular call will be the final value of the variable on exit from the function body.
The convention has been widely imitated, starting with Delphi and most recently in Microsoft’s “code contracts”, a kind of poor-man’s Design by Contract emulation, achieved through libraries; it requires a Result notation to denote the function result in a postcondition, although this notation is unrelated to the mechanisms in the target languages such as C#. As the example of Eiffel’s design illustrates, a programming language is a delicate construction where all elements should fit together; the Result convention relies on many other essential concepts of the language, and in turn makes them possible.
Reference
[1] Eiffel Software discussion group, here.