F# records vs .net struct

Question

is f# records is the same as .net struct? I saw people talk about f# struct,are they use this term interchangable with F# records? Like in FSharp runs my algorithm slower than P

Answer 1

No, in fact, a record type in F# is a reference type just with special functional programming features like pattern matching on properties, easier immutability, and better type inference.

I think Laurent's speedup must have been for other reasons, because we can prove that Tup is not a ValueType:

type Tup = {x: int; y: int}
typeof<Tup>.BaseType = typeof<System.Object> //true

Whereas

type StructType = struct end
typeof<StructType>.BaseType = typeof<System.ValueType> //true
typeof<StructType>.BaseType = typeof<System.Object> //false

Answer 2

As Stephen said, it is a reference type. Here is the compiled code(release mode) of type Tup = {x: int; y: int}:

[Serializable, CompilationMapping(SourceConstructFlags.RecordType)]
public sealed class Tup : IEquatable<xxx.Tup>, IStructuralEquatable, IComparable<xxx.Tup>, IComparable, IStructuralComparable
{
    // Fields
    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    internal int x@;
    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    internal int y@;

    // Methods
    public Tup(int x, int y);
    ...

    // Properties
    [CompilationMapping(SourceConstructFlags.Field, 0)]
    public int x { get; }
    [CompilationMapping(SourceConstructFlags.Field, 1)]
    public int y { get; }
}

Answer 3

To answer the second part of the question, it's really about using them as a dictionary key.

The speed of using something as a dictionary key comes down to how the GetHashCode function works for that type. For reference types such as Records, the default .Net behaviour uses Object.GetHashCode which "computes the hash code based on the object's reference" which is an efficient numeric operation.

The more complex default behaviour for value types, which is what structs are is ValueType.GetHashCode "method of the base class uses reflection to compute the hash code based on the values of the type's fields." so the more complex the struct and depending on its fields, computing the hash could take a lot longer.

Answer 4

Stephen Swensen is correct, I'd like to add one important detail, Record Types support structural equality, as do .NET’s value type, so there equality tests have the similar behavior (there may be subtle differences if a record contains something that can’t be compared using structural equality).