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--- confusing:lyahpop [2021/03/23 03:10] – created f2b216
+++ confusing:lyahpop [2025/10/08 00:48] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
+====== 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
-<WRAP center round box 60%>
+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 >
+<code Haskell>
-    import Control.Monad.State
+import Control.Monad.State
-    stackManip :: State Stack Int
+stackManip :: State Stack Int
-    stackManip = do
+stackManip = do
-        push 3
+    push 3
-        a <- pop
+    a <- pop
-        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'.
+   |
+| 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!