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jun3/haskell: fix questions and add some new #341

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jun3/haskell: fix questions and add some new
antonkalinin-ml Dec 21, 2021
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26 changes: 18 additions & 8 deletions backend/junior-3/haskell.md
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Expand Up @@ -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`
Expand Down Expand Up @@ -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?
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@evgeny-osipenko evgeny-osipenko Dec 22, 2021

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Вопрос "как определяется ранг функции", на мой взгляд - вполне корректный и не нуждается в удалении. Можно пояснить, почему он заменяется на "что такое функция высшего ранга"? Это более слабый вопрос, поскольку в нём не уточняется, например, чем именно отличается функция 2-го ранга от функции 3-го.

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@antonkalinin-ml antonkalinin-ml Dec 22, 2021

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Чтобы определять числовой ранг функции, у нас не хватает источника. В доках по хаскелю не объяснено, как по сигнатуре определить ранг.

Стас говорил, что определение ранга непростое, и мы подумали, что оно на практике не нужно. Достаточно отличать первый ранг от всех остальных, а уж какой он - третий или четвертый - не влияет на то, как применять RankNTypes.

* 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`?
Expand Down Expand Up @@ -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
Expand All @@ -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`?
Expand Down Expand Up @@ -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.

## Lists

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