I am trying to add a string to the end of a string in Haskell.
albumStr = ""
main = do
let albumStr = albumStr ++ "nothing"
print albumStr
Whenever i run this it just gets stuck on " in the console and i have to terminate it.
Why? and how do i add a string to another string in this way?
Edit: How do i add multiple strings to the end of a current string without overwritting it.
Thanks
Unlike ML, Haskell does not have a rec keyword to mark recursive definitions. Instead all definitions can be recursive, meaning that in every variable definition, the defined variable is already in scope during its definition.
So in let albumStr = albumStr ++ "nothing", the albumStr on the right of the = refers to the one defined on the left of the = - not the one defined in line 1. Therefore the definition is infinitely recursive and loops forever.
If you want to define a variable based on another one, you have to give it a different name:
let localAlbumStr = albumStr ++ "nothing"
let-binding. but not if it's a do-binding.The equation
albumStr = albumStr ++ "nothing"
defines a string recursively, and does not use the globally defined albumStr at all. The recursion hangs immediately (it similar to an infinite loop).
Turning on warnings reports the shadowing of the global name.
If you do not want a recursive definition, use another variable name:
albumStr2 = albumStr ++ "nothing"
let nonrec -- the non-recursive / shadowing definition let x = f x in ... is not that common (at least in Haskell). This is a very minor point, though.albumStr ++ concat listOfStrings. Remember that you can't modify the global value of albumStr (probably you don't even need that), but only create a new string.In do notation, you can write
do { albumStr <- pure (albumStr ++ "nothing")
; ...
}
and it would (seem to) work.
While let (and function) bindings in Haskell are recursive, do bindings are not.
Instead, a shadowing variable is automatically created, which is actually a new variable with the same name. The code to the right of <- lies in the scope above the binding; the code to the left of <- is in a new, wider scope (that includes the earlier one).
See also:
let str = str ++ "nothing", Haskell reads that as "Construct a new value calledstr, which has the property thatstr = str ++ "nothing"," in other words a never-ending recursive computation.