Why does Haskell not have records with structural typing?

Question

I have heard Haskell described as having structural typing. Records are an exception to that though as I understand. For example foo cannot be called with something of type HRec2 even though HRec and HRec2 are only nominally different in their fields.

data HRec = HRec { x :: Int, y :: Bool }
data HRec2 = HRec2 { p :: Int, q :: Bool }

foo :: HRec -> Bool

Is there some explanation for rejecting extending structural typing to everything including records?

Are there statically typed languages with structural typing even for records? Is there maybe some debate on this I can read about for all statically typed languages in general?

Answer 1

Haskell has structured types, but not structural typing, and that's not likely to change.*

The refusal to permit nominally different but structurally similar types as interchangeable arguments is called type safety. It's a good thing. Haskell even has a newtype declaration to provide types which are only nominally different, to allow you to enforce more type safety. Type safety is an easy way to catch bugs early rather than permit them at runtime.

In addition to amindfv's good answer which includes ad hoc polymorphism via typeclasses (effectively a programmer-declared feature equivalence), there's parametric polymorphism where you allow absolutely any type, so [a] allows any type in your list and BTree a allows any type in your binary tree.

This gives three answers to "are these types interchangeable?".

1. No; the programmer didn't say so.
2. Equivalent for a specific purpose because the programmer said so.
3. Don't care - I can do the same thing to this collection of data because it doesn't use any property of the data itself.

There's no 4: compiler overrules programmer because they happened to use a couple of Ints and a String like in that other function.

*I said Haskell is unlikely to change to structural typing. There is some discussion to introduce some form of extensible records, but no plans to make (Int,(Int,Int)) count as the same as (Int, Int, Int) or Triple {one::Int, two::Int, three::Int} the same as Triple2 {one2::Int, two2::Int, three2::Int}.