====== Confusing pop statement in LYAH ======
~~DISCUSSION~~

Nothing is perfect. Even though a tremendous effort must have been taken to create LYAH )<sup>1</sup> I found it very confusing - for a long time. Since, I understand Haskell more and more I just realised that there is also code
  - which does not really make sense
  - hence, generates warnings
  - it also generates compiler errors, because of incompatibilities with new packages philosophies
  - can generate runtime errors if the code would be extendend

LYAH )<sup>1</sup> is stating [[http://learnyouahaskell.com/for-a-few-monads-more#state|here]]:


<WRAP center round box 62%>

<code Haskell>
import Control.Monad.State  

pop :: State Stack Int  
pop = State $ \(x:xs) -> (x,xs)  

push :: Int -> State Stack ()  
push a = State $ \xs -> ((),a:xs)  
</code>

<WRAP center round box 100%>
pop is already a stateful computation and push takes an Int and returns a stateful computation. Now we can rewrite our previous example of pushing 3 onto the stack and then popping two numbers off like this:
</WRAP>

<code Haskell>
import Control.Monad.State  

stackManip :: State Stack Int  
stackManip = do  
    push 3  
    a <- pop  
    pop
</code>

</WRAP>

  - The line ''a <- pop'' is "assigning" a state to a binding ("variable") a but a will never be picked up, and the with just ''pop'' discards the result completely.
  - Hence, it creates compiler warning:
    * <code>
app\Main.hs:28:5: warning: [-Wunused-matches]
    Defined but not used: `c'
   |
mm |     c <- pop
   |     ^
app\Main.hs:33:5: warning: [-Wunused-do-bind]
    A do-notation statement discarded a result of type `Int'
    Suppress this warning by saying `_ <- pop'
   |
nn |     pop
   |     ^^^
Linking .stack-work\...
</code>
  - The new package philosophy renamed the data type State to StateT (The StateT monad transformer).
    * <code>
app\Main.hs:21:10: error:
    Not in scope: data constructor `S.State'
    Perhaps you meant one of these:
      `S.StateT' (imported from Control.Monad.State),
      variable `S.state' (imported from Control.Monad.State)
    Module `Control.Monad.State' does not export `State'.
   |
21 | push a = S.State $ \xs -> ((),a:xs)
   |          
</code>
  - If you use to much ''pop''s in ''stackManip'' then you get the following __**runtime**__ error:
    * ...-exe...: app\Main.hs:xx:yy-zz: Non-exhaustive patterns in lambda
      * which is this line ''pop = S.state $ \(x:xs) -> (x,xs)''

----


LYAH )<sup>1</sup> Learn You a Haskell for Great Good!