How to assign to mvar with case syntax? [closed]

Question

Closed. This question needs details or clarity. It is not currently accepting answers.

---

Want to improve this question? Add details and clarify the problem by editing this post.

Closed 5 years ago.

I'am asking if i can add same think like that :

if maVar == "false" then maVar = "" else maVar = maVar

in tmy code haskell (yesod)

let maVar= unpack $ case Map.lookup "maVarSession" sess of
        Just a -> a
        Nothing -> "Nothing"

Answer 1

No, maVar is imutable and it is String always.

In Haskell there aren't any false. It has False.

You could do next:

let maVar = ...
in
  let maVarNew = if maVar == "Nothing" then "" else maVar
  in

Answer 2

It appears you're attempting to assign a new value to maVar. This does not make sense in Haskell, because Haskell doesn't really have mutable variables,* and you're getting a syntax error because the parser sees = in a position which does not accept declarations.

Instead, you can declare a new value (we can name it whatever, but let's call it maVar'), defined maVar' = if maVar == "false" then "" else maVar.

Then, we can write

let maVar = unpack $ case Map.lookup "maVarSession" sess of
        Just a -> a
        Nothing -> "Nothing"
    maVar' = if maVar == "false" then "" else maVar
  [...]

Guards can be also used to write the new declaration (perhaps more clearly, possibly not) as

    maVar'
      | maVar == "false" = ""
      | otherwise = maVar

It would probably be a good idea to make sure you understand what exactly you're doing, however; perhaps read some introductory texts on Haskell to understand purity and how Haskell works differently from languages you might be familiar with.

* You can have a mutable cell, but you really have to ask specifically for it—you can't easily do it by accident.