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94 changes: 63 additions & 31 deletions backend/middle-1/haskell.md
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Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,7 @@
<details>
<summary>List of contents</summary>

- [Basic type classes](#basic-type-classes)
- [Functors](#functors)
- [Type classes](#type-classes)
- [Types](#types)
- [Polymorphic Kinds](#polymorphic-kinds)
Expand All @@ -19,34 +19,40 @@
- [Kata](#kata)

</details>

## Basic type classes

* Functors
* What is covariance and contravariance in the context of functors and category theory?
* What are the negative and positive positions?
* What types can be both covariant functors and contravariant?
* What are invariant functors?
* Contrafunctor (Contravariant functor)
* What is the signature of `contramap` function?
* Make an example of instance definition for some ADT.
* What is the semantic meaning of applying `contramap`?
* Bifunctor
* What is the signature of `bimap` function?
* What ADT's do have the `Bifunctor` instance?
* Is a bifunctor covariant or contravariant on type variables applied to it?
* Profunctor
* What is the signature of `dimap` function?
* Write an instance implementation for `(->)`.
* Name a few practical use cases (at least one).
* Which of type variables applied to a profunctor appear in negative and which in positive position?
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Тут ничего не изменилось, кроме заголовка и убирания одного уровня отступа под элементом Functors.


## Functors

* What is covariance and contravariance in the context of functors and category theory?
* What are the negative and positive positions?
* What types can be both covariant functors and contravariant?
* What are invariant functors?
* Contrafunctor (Contravariant functor)
* What is the signature of `contramap` function?
* Make an example of instance definition for some ADT.
* What is the semantic meaning of applying `contramap`?
* Bifunctor
* What is the signature of `bimap` function?
* What ADT's do have the `Bifunctor` instance?
* Is a bifunctor covariant or contravariant on type variables applied to it?
* Profunctor
* What is the signature of `dimap` function?
* Write an instance implementation for `(->)`.
* Name a few practical use cases (at least one).
* Which of type variables applied to a profunctor appear in negative and which in positive position?

#### Resources

* Functors
* [(Co-contra) variance](https://www.fpcomplete.com/blog/2016/11/covariance-contravariance)
* [I love profunctors](https://www.schoolofhaskell.com/school/to-infinity-and-beyond/pick-of-the-week/profunctors)
* [Functoriality (just not bad article about functors)](https://bartoszmilewski.com/2015/02/03/functoriality/)
* [(Co-contra) variance](https://www.fpcomplete.com/blog/2016/11/covariance-contravariance)
* [I love profunctors](https://www.schoolofhaskell.com/school/to-infinity-and-beyond/pick-of-the-week/profunctors)
* [Functoriality (just not bad article about functors)](https://bartoszmilewski.com/2015/02/03/functoriality/)
* An example of using profunctors:
[`Statement`](https://hackage.haskell.org/package/hasql-1.5.0.2/docs/Hasql-Statement.html#t:Statement)
type from the `hasql` library.
* typeclasses.com on [contravariance](https://typeclasses.com/contravariance)
* typeclasses.com on [profunctors](https://typeclasses.com/profunctors)
* An easy explanation of `Contravariant`: a `Functor` may be thought as it
"contains" or "produces" values, while a `Contravariant` "consumes" them. Pay
attention on the polarity term.
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@antonkalinin-ml antonkalinin-ml Mar 24, 2022 •
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Я обещал добавить источник, где я видел это объяснение, но не смог его найти, так что пришлось написать прямо здесь. Ребятам на сдаче понравилось. После прочтения других источников оно не всегда появляется в голове.


## Type classes

Expand Down Expand Up @@ -97,7 +103,7 @@
* What are Pattern Synonyms and what are they used for?
* Can we pattern match on concrete values (constants) in the right-hand side?
* Where can Pattern Synonyms occur? Can we declare them locally, inside of functions?
* What is the restriction on type variables with Bidirectional Pattern Synonyms?
* What is the restriction on parameters of Bidirectional Pattern Synonyms?
* What are Unidirectional Pattern Synonyms?
What are their constraints comparing with Bidirectional Pattern Synonyms?
* What are Explicitly Bidirectional Pattern Synonyms?
Expand Down Expand Up @@ -141,15 +147,34 @@ How can we bundle Pattern Synonyms with datatypes in export and import lists?

## Laziness

* How is value from evaluated thunk stored (are we allowed to avoid redundant reevaluations)?
* Enumerate cases where thunk with ADT will be evaluated.
* What is the irrefutable pattern and how does it work?
* What does the `sprint` function do?
* Describe the process of evaluating a thunk and storing the evaluated value.
* After a thunk is fully evaluated, can the GHC runtime evaluate it again?
* What is a black hole and what problem does it solve?
* For what kind of thunks are black holes used?
* Enumerate cases where a thunk returning an ADT is evaluated.
* What is an irrefutable pattern and how does it work?
* Where are patterns in Haskell irrefutable by default?
* Which patterns are irrefutable:
1. `f (Just a) = ...`
1. `let (Just a) = ...`
1. `where (Just a) = ...`
1. `g (N a) = ...` where `newtype N a = N a`.
1. `g (D a) = ...` where `data D a = D a`.
1. `f a = ...`
1. `f _ = ...`
* When can irrefutable patterns be useful? Hint: you may describe why
[`Data.List.partition`](https://hackage.haskell.org/package/base-4.16.0.0/docs/src/Data.OldList.html#partition)
uses them.
* What does the `sprint` command do in `ghci`?

#### Resources

* [Laziness from What I Wish I Knew When Learning Haskell](http://dev.stephendiehl.com/hask/#laziness)
* [The GHC Runtime System - Ch. 4 Laziness](http://ezyang.com/jfp-ghc-rts-draft.pdf)
* Haskell 2010 Language Report:
* See p. 3.17.2 Informal Semantics of Pattern Matching in [Pattern Matching](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-580003.17)
* [Irrefutable Patterns in Let Expressions](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-440003.12)
* See p. 4.4.3.2 Pattern bindings in [Nested declarations](https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-800004.4)

## Exceptions

Expand All @@ -164,6 +189,8 @@ How can we bundle Pattern Synonyms with datatypes in export and import lists?
Why do we need it?
* What is the purpose of `safe-exceptions` library? Which exception handling problems does it address?
Why is `unliftio` considered safer by the author of `safe-exceptions`?
* When does the problem of rethrowing asynchronous exceptions as synchronous
happen and how can it be solved?
* Describe a problem which arises when handling exceptions and using functions like `bracket` with stateful monadic stacks.
* How is it solved in `monad-control` library?
* How is it solved in `unliftio` library?
Expand Down Expand Up @@ -235,13 +262,18 @@ What are the commands which help with that?
* What is a `Traversal`?
* What is an `Iso`?
* Why is `Monoid` constraint required in `view` for traversals?
* Is a `Prism` a `Lens`, a `Traversal` a `Lens`, an `Iso` a `Lens`, a
`Traversal` an `Iso`, a `Prism` an `Iso`, or vice versa?
* Is the `traverse` method of the `Traversable` class a `Traverse`?
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a Traverse -> a Traversal

* What are the lens laws?
* Why do lenses fit well for composing?
* How operators are grouped by name (which ones are started with `^`, which ones contain `~`, `.` (dot), `%`, `=`)?
* What combinators are purposed for working in `State` monad?
Why is it convenient?
* What is the goal of the microlens library?
When to use it and when do not?
* When and why is it better to use `generic-lens` and `optics` libraries? What
libraries can we use together?
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Мне кажется нужно отдельно сделать этот вопрос:
What libraries can we use together (lens, generic-lens, microlens, optics) ?


#### Resources

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