how does [<Literal>] differ from other constants in F#

Question

I am a bit confused by the Literal keyword and why it is necessary in F#.

Reading the docs, it sounds to me that [<Literal>] is used to define a constant, however I am a bit confused how this constant differs from all other constants in F#..

Values that are intended to be constants can be marked with the Literal attribute. This attribute has the effect of causing a value to be compiled as a constant.

When I think of a constant, I think of something which is immutable....

let x = "a" + "b" //this is a immutable value, its value is constant [<Literal>] let y = "a" + "b" //this is also a immutable value, but why is this a special constant? 

Is it because the 'normal' F# values are evaluated lazily and the [<Literal>] is not evaluated lazily..? is that what they mean with 'compiled as constant'..? or is there something else to it?

Answer 1

In your example, x is an immutable value that is assigned during runtime (but NOT evaluated lazily), whereas y is assigned during compiletime. As an example,

let myDLL = "foo.dll"  [<DllImport(myDLL, CallingConvention = CallingConvention.Cdecl)>] extern void HelloWorld() 

will not work, because DllImport is an attribute and needs to know the value of myDLL during compilation. However, this will work:

[<Literal>] let myDLL = "foo.dll"  [<DllImport(myDLL, CallingConvention = CallingConvention.Cdecl)>] extern void HelloWorld() 

Answer 2

If you come from C# background, you can think of Literal values as const fields, and non-literal ones as readonly fields. The same differences apply.