Swift Difference Between Double and Float64

Question

In Swift Programing language official documentation, It says

Double represents a 64-bit floating-point number.
Float represents a 32-bit floating-point number.

Link: https://docs.swift.org/swift-book/LanguageGuide/TheBasics.html#ID321

Then, Why is there Float64? What is the difference between them? Or Are they same?

Answer 1

The headers, found by hitting command+shift+o and searching for Float64, say:

/// A 64-bit floating point type.
public typealias Float64 = Double
/// A 32-bit floating point type.
public typealias Float32 = Float

and

Base floating point types 

    Float32         32 bit IEEE float:  1 sign bit, 8 exponent bits, 23 fraction bits
    Float64         64 bit IEEE float:  1 sign bit, 11 exponent bits, 52 fraction bits  
    Float80         80 bit MacOS float: 1 sign bit, 15 exponent bits, 1 integer bit, 63 fraction bits
    Float96         96 bit 68881 float: 1 sign bit, 15 exponent bits, 16 pad bits, 1 integer bit, 63 fraction bits

Note: These are fixed size floating point types, useful when writing a floating
      point value to disk.  If your compiler does not support a particular size 
      float, a struct is used instead.
      Use of of the NCEG types (e.g. double_t) or an ANSI C type (e.g. double) if
      you want a floating point representation that is natural for any given
      compiler, but might be a different size on different compilers.

As a general rule, unless you’re writing code that is dependent on binary representations, you should use the standard Float v Double names. But if you are writing something where binary compatibility is needed (e.g. writing/parsing binary Data to be exchanged with some other platform), then you can use the data types that bear the number of bits in the name, e.g. Float32 vs. Float64 vs. Float80.

Answer 2

Go to the definiation of Float64

/// A 64-bit floating point type.
public typealias Float64 = Double

/// A 32-bit floating point type.
public typealias Float32 = Float