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I have a function that return [[]], and I want to test the result as unit test. But I found that the expression [[]] == [[]] return false. Here a simple test code:

# [[]] == [[]];;
- : bool = false

Can someone explain me why this expression is evaluated as false?

Thanks.

pad
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Atikae
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  • There is more information on structural and physical equality in another question, http://stackoverflow.com/questions/1412668/does-have-meaning-in-ocaml/ – nlucaroni Apr 04 '12 at 21:30

2 Answers2

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Use = since you have structural equality for comparing two values: 

# [[]] = [[]];;
- : bool = true

Because == is reference equality, it only returns true if you refer to the same memory location:

let a = [[]]
let b = a

# b == a;;
- : bool = true
pad
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9

The == operator in OCaml means "physical equality". However, you have two (physically) different lists. Probably, you want "structural equality", which is tested by =.

Ellie Kesselman
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Matthias
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  • Ok I undertstand now. But it means that the first [[]] is 'a list list and the second 'b list list ? – Atikae Apr 04 '12 at 11:02
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    Yes, each `[]` allocates a new list (c.f. http://www.cs.jhu.edu/~scott/pl/lectures/caml-intro.html), but both list are not the *very same* list. – Matthias Apr 04 '12 at 11:11
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    `[]` has structural and physical equality (it's integer-like). It does not allocate a new list. It's the outer brackets that are creating the new list, since, `[[]] = ([] :: [])`. – nlucaroni Apr 04 '12 at 21:35
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    @nlucaroni: Actually, I referred to the outer brackets. Thank you to clear this point. – Matthias Apr 05 '12 at 03:50