I saw the following # sign in some libraries and don’t know what its meaning is:
let Compile (e: #Expr)
Expr refers to FSharp.Quotations.Expr in this case - not sure if this is relevant.
Expr
#Expris a shortcut for 'T when 'T :> Exprwhich can be read as
#Expr
'T when 'T :> Expr
some type, which we will call now T, which is compatible with the typeExpr
T
MSDN has additional documentation on Flexible Types .
See also. What is the purpose of flexible type annotation in F #? here in a stack overflow.