Merge pull request #341 from fullstack-development/jun3-haskell-fixes

jun3/haskell: fix questions and add some new

Author: stanislav-az

backend/junior-3/haskell.md

@@ -44,7 +44,7 @@
   * [Introduction to Tagless Final](https://serokell.io/blog/2018/12/07/tagless-final)
 
 
-## TypeOperators and type classes extensions:
+## TypeOperators and type classes extensions
 
 * `TypeOperators`
 * `FlexibleContexts` & `FlexibleInstances`
@@ -127,7 +127,7 @@
 * `ScopedTypeVariables`
   * What is the main goal of this extension?
 * Higher ranked types
-  * What is a "rank" of a function?
+  * What is a higher rank function?
   * Give examples of rank-1, rank-2, rank-3 functions.
   * What is the main goal of `RankNTypes` extension?
   * Can we create datatypes and newtypes with `RankNTypes`?
@@ -197,18 +197,24 @@
   * `1:(thunk)`
   * `1:2:(thunk)`
   * `1:2:3:[]`
+  * `thunk:thunk`
   * `15`
   * `\x -> x * 2`
   * `(\x -> x + 1) 3`
 * Can haskell evaluate in strict mode?
-* Why strict functions in Haskell evaluate values to WHNF and not NF?
+* Why are values in Haskell typically (when using `Strict`, `seq`, `foldl'`,
+  `modifyIORef'` etc) calculated to WHNF, not to NF?
 * What is the function `seq` (and operator `$!`)?
 * What is the function `deepseq` (and operator `$!!`)?
 * Could using `seq` change the returned value of the function?
+* In which way does runtime behavior of the following expressions differ?
+  * ``a + b `seq` (a + b) : list``
+  * ``let s = a + b in s `seq` s : list``
+* What are conditions when `seq` or `deepseq` can actually evaluate a value?
 * What is the GHC extension `BangPatterns`?
-  * Make examples when bang pattern is useless.
-  * Make examples when bang pattern has less power, than it could be supposed.
-  * Show the difference between this two definitions:
+  * Make examples when a bang pattern is useless.
+  * Do bang patterns force values to WHNF or NF?
+  * Show the difference between these two definitions:
     * `f1`
 
     ```haskell
@@ -223,11 +229,13 @@
     f2 x False = False
     ```
 
-  * Does bang patterns force execution, when they are nested in constructors inside of `let` or `where` expressions?
-    * There is a `!` inside a `Maybe`, which is inside a `let` expression, and `let` expressions are lazy:
+  * Do bang patterns force execution when they are nested in constructors inside
+    `let` or `where` expressions? Does this force `a` to be evaluated or when
+    it does?
 
     ```haskell
     let (Just !a) = x in 1 + 1
+    let (Just !a) = x in [a]
     ```
 
 * What are the GHC extensions `Strict` and `StrictData`?
@@ -260,6 +268,8 @@
   * [Brief normal forms explanation with Haskell - Medium article](https://medium.com/@aleksandrasays/brief-normal-forms-explanation-with-haskell-cd5dfa94a157)
   * [All About Strictness. - FP Complete](https://www.fpcomplete.com/blog/2017/09/all-about-strictness)
   * [Does a function in Haskell always evaluate its return value? - Stackoverflow](https://stackoverflow.com/questions/27685224/does-a-function-in-haskell-always-evaluate-its-return-value)
+  * See "Learning to use `seq`",
+    [Real World Haskell](http://book.realworldhaskell.org/read/functional-programming.html) book.
 
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