Can I include formulas in a struct somehow?

Question

I am trying to create a struct which uses a formula to automatically create data in one of the struct fields when the other two values are entered.

For example, I want to create a 2D rectangular room with Length and Width which are values that are entered. I would then like to include the formula Area = Length * Width in the struct.

Have tried and just get a syntax error :

syntax error: unexpected =, expecting semicolon or newline or }

// CURRENT CODE
type room struct {
 L int
 W int
 A int
}
// WOULD LIKE IT TO BE
type room struct {
 L int
 W int
 A int = room.L*room.H
}

Go does not allow you to set defaults as part of the struct definition. See this post for strategies to deal with this: https://stackoverflow.com/questions/37135193/how-to-set-default-values-in-go-structs — kingkupps, Oct 27 '19 at 08:24

score 2 · Answer 1 · answered Oct 27 '19 at 08:26

2

Since A is invariant, this would be a good fit for a function, not a field.

type room struct {
    L int
    W int
}

func (r *room) area() int {
    return r.L * r.W
}

answered Oct 27 '19 at 08:26

Emily

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Thanks, yes I was aware of that, was just wondering if it was at all possible to do it another way. I cannot understand why that is not possible. – Oct 27 '19 at 12:17

score 1 · Answer 2 · answered Oct 27 '19 at 09:01

1

If you would like to keep A as a Field, you can optionally preform the computation in a constructor.

type room struct {
 L int
 W int
 A int
}

func newRoom(length, width, int) room {
  return room{
    L: length,
    W: width,
    A: length * width,
  }
}

answered Oct 27 '19 at 09:01

Simon

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1

Thanks, was just wondering if there was any way of making it happen within the struct so as not to add unnecessary code. – Oct 27 '19 at 12:17
1

You may be able to use some hacky struct tags to do that, but it's not very Go-like or recommended. – Simon Oct 27 '19 at 18:42

kostix · Answer 3 · 2019-12-31T14:41:24.450

If you think about what you're after, you'll see that basically your desire to "not add unnecessary code" is really about not writing any code by hand, rather than not executing any code: sure, if the type definition

type room struct {
 L int
 W int
 A int = room.L*room.H
}

could be possible in Go, that would mean the Go compiler would have make arrangements so than any code like this

var r room
r.L = 42

is compiled in a way to implicitly mutate r.A.

In other words, the compiler must make sure that any modification of either L or W fields of any variable of type room in a program would also perform a calculaton and update the field A of each such variable.

This poses several problems:

What if your formula is trickier—like, say, A int = room.L/room.W?

First, given the casual Go rules for zero values of type int, an innocent declaration var r room would immediately crash the program because of the integer division by zero performed by the code inserted by the compiler to force the invariant being discussed.

Second, even if we would invent a questionable rule of not calculating a formula on mere declarations (which, in Go, are also initializations), the problem would remain: what would happen in the following scenario?
```
var r room
r.L = 42
```
As you can see, even if the compiler would not make the program crash on the first line, it would have to arrange for that on the second.

Sure, we could add another questionable rule to sidestep the problem: either somehow "mark" each field as "explicitly set" or require the user to provide an explicit "constructor" for such types "armed" with a "formula". Either solution stinks in its own way: tracing write field access incurs performance costs (some fields now have a hidden flag which takes up space, and each access of such fields spends extra CPU counts), and having constructors goes again one of the cornerstone principles of the Go design: to have as little magic as possible.
The formula creates a hidden write.

This may not be obvious until you start writing "harder-core" Go programs for tasks it shines at—highly concurrent code with lots of simultaneously working goroutines,—but when you do you're forced to think about shared state and the ways it's mutated and—consequently—on the ways such mutations are synchronized to keep the program correct.

So, let's suppose we protect access to either W or L with a mutex; how would the compiler make sure mutation of A is also proteted given that mutex operations are explicit (that is, a programmer explicitly codes locking/unlocking operations)?
(A problem somewhat related to the previous one.) What if "the formula" does "interesting things"—such as accessing/mutating external state?

This could be anything from accessing global variables to querying databases to working with a filesystems to exchanges over IPC or via networking protocols.

And this all could be very innocently-looking, like A int = room.L * room.W * getCoefficient() where all the nifty details are hidden in that getCoefficient() call.

Sure, we, again, could work-around this by imposing an arbitrary limit on the compiler to only allow explicit access to the fields of the same enclosing type and only allow them to participate in simple expressions with no function calls or some "whitelisted" subset of them such as math.Abs or whatever. This clearly reduces the usefulness of the feature while greatly complicating the language.
What if "the formula" has non-linear complexity?

Suppose, the formula is O(N³) with regard to the value of W.

Then setting W on a value to 0 would be processed almost instantly but setting it to 10000 would slow the program down quite noticeably, and both of these outcomes would result form a seemingly not too different statements: r.W = 0 vs r.W = 10000.

This, again, goes agains the principle of having as little magic as possible.
Why would we ony allow such things on struct types and not on arbitrary variables—prodived they are all in the same lexical scope?

This looks like another arbitrary restriction.

And another—supposedly—the most obvious problem is what should happen when the programmer goes like

var r room
r.L = 2  // r.A is now 2×0=0
r.W = 5  // r.A is now 2×5=10
r.A = 42 // The invariant r.A = r.L×r.W is now broken

?

Now you can see that all the problems above may be solved by merily coding what you need, say, with the following approach:

// use "unexported" fields
type room struct {
 l int
 w int
 a int
}

func (r *room) SetL(v int) {
  r.l = v
  updateArea()
}

func (r *room) SetW(v int) {
  r.w = v
  updateArea()
}

func (r *room) GetA() int {
  return r.a
}

func (r *room) updateArea() {
  r.a = r.l * r.w
}

With this approach, you may be crystal-clear about all the issues above.

Remember that the programs are written for humans to read and only then for machines to execute; it's paramount for proper software engeneering to keep the code as much without any magic or intricate hidden dependencies between various parts of of it as possible. Please remember that

Software engineering is what happens to programming when you add time and other programmers.

© Russ Cox

Thanks, a lot more detailed than I can understand though do get the idea of what you mean. It is appreciated, I am new to learning Go, and programming in general and any input helps. — , Jan 01 '20 at 15:28

Can I include formulas in a struct somehow?

3 Answers3