Questions tagged [haskell]
Haskell is a functional programming language featuring strong static typing, lazy evaluation, extensive parallelism and concurrency support, and unique abstraction capabilities.
76 questions
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Haskell instance resolution on recursive types
Not sure if this question fits here, but I'm looking to implement (or try to) Hakell's type classes into my own language, however I don't understand how resolution works on recursive types like the ...
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Is Calling a Function a Side Effect
I've noticed a pattern in trying to make functional programming effective - there is still some kind of impure, effectful operation going on, but it gets holed up in a single, manageable imperative ...
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Is there any reference materials on complexity analysis for lazy languages?
Is there any books, papers or articles on how to analyze the time complexity of programs written in lazy languages such as Haskell?
I know how laziness is implemented and how it can be expanded and ...
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Representation of pairs in System F
System F defines the data type pair as:
$$X\times Y := \Pi Z. (X\to Y \to Z)\to Z$$
with:
$$\langle x,y \rangle := \Lambda Z. \lambda p^{X\to Y\to Z}.p \text{ }x\text{ } y$$
Projections are defined:
$$...
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DFA for even concatenation of strings from a language
If I have a deterministic finite automaton (DFA) with a language $W$, and I need to create another DFA that returns all the strings that are a concatenation of an even number of strings in $W$, how ...
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Are Haskell monads stronger than strong monads?
Haskell's monads are usually considered to mean strong monads in category theory, but it seems like the former is a bit stronger than the latter. With strong monads, you have a Kleisli extension ...
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Mathematical explanation of the usage of monads to wrap data for avoiding errors
For the past two days I have been on a question to understand monads in the context of Haskell. A nice explanation I found is by Graham Hutton on the Computerphile channel see here. This explanation ...
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How much is decidability compromised within this restriction of the fixpoint combinator?
Though purely functional programming languages, such as Haskell, is commonly thought to have no side-effects, there is a caveat: Recursive calls may hang.
I considered this to be undesirable, and ...
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Literature on delta encoding serializeable ADTs
Suppose that I have some nested algebraic data type (ie. something one can construct via datas in Haskell) that is serializeable (so no functional fields ...
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What is the type of a type signature?
For example, using GHCi,
ghci> f x = x + 1
ghci> :t f
f :: Num a => a -> a
What is the type of the type signature ...
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Strictness in both arguments but not in each individually
I'm learning about strict functions in Haskell.
A function f is strict if f ⊥ = ⊥
Some functions are strict only in the first argument (for e.g. const), others are strict in the second (for e.g. map)....
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Book references for combinatory logic as applied in Haskell?
I am looking for book references on combinatory logic. Is there a book focused on how combinatory logic is applied in the context of pure functional languages like Haskell?
I found "Combinators: ...
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Ambiguous type of "triangle" operator for sum types
In Meijer, Fokkinga and Patersons "Functional Programming with Bananas, Lenses, Envelopes and Barbed Wire" the ∇ operator for sum types is introduced which removes the tags from its ...
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Is type inference for arbitrary-rank types decidable when supplied type signatures?
I found following statements in 6.4.16. Arbitrary-rank polymorphism of ghc document.
GHC uses an algorithm proposed by Odersky and Laufer (“Putting type annotations to work”, POPL‘96) to get a ...
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The second Functor law is redundant, but I don't understand the proof
When we defining a Functor instance in Haskell, it should satisfy the following two laws:
fmap id = id
...
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How does GHC insert type abstraction/application under the RankNTypes extension
I'm developing a functional programming language that offers Rank-n polymorphism. Like Haskell I don't want types to appear at the term level, but I have no idea to insert type abstraction and type ...
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What exactly is the relation between Haskell and category theory?
In articles or Quora posts about category theory, I often find mentions of the programming language Haskell. I have little knowledge of category theory and even less of programming. Could someone ...
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Algorithm for Getting Largest Connected Component From List of Touching Pairs
I have a program which finds the touching pairs of a given value in a binary image. For example, consider the below image:
...
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What is the meaning of "You describe the result you want rather than specifying the steps required to get there." in Functional Programming?
One of the characteristics of functional programming is as follows:
You’re describing the result you want rather than explicitly specifying the steps required to get there.
I found this quote at ...
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Implementing a correct `let rec in` interpreter in Haskell with `eval` and `sub` semantics
I'm implementing the toy fb-lang from the principles of programming language book. It's a small interpreted language that uses eval(uate) expression function and <...
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Pure Directed Graph
How can a directed graph be efficiently represented in a purely functional language like Haskell? Could someone suggest relevant materials on this topic? (functional pearls perhaps?) Thanks.
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GroupBy key a sequence of ordered key values
I have a sorted (by key) sequence of key value pairs:
...
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Can we somehow get functoriality from purely type-theoretic reasoning?
In this question, I asked about how to prove naturality from parametric polymorphism, using parametricity. The current answer to that question simply assumes that the functors in question satisfy the ...
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Rigorous proof that parametric polymorphism implies naturality using parametricity?
This question asks for an informal explanation of why all polymorphic functions between functors are natural transformations (This is a claim made by Bartosz Milewski). One answer to that question ...
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λ -terms that correspond to Haskell functions
Evening All
I already have a "grasp" of haskell (not terrible, about 6 month experience) and am trying to learn the fundamentals that sit behind it, thus am now turning my attention to ...
user130129
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Show that term cons works by showing all beta reductions
I'm new to functional programming.
So the terms cons appends an element to the front of the list. Where cons ≜ λx:λl:λc:λn: c x (l c n).
How should I go about proving that cons works correctly using ...
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Monad in Haskell programming vs. Monad in category theory
I have a question about concept of monad used in Haskell programming
and category theory in math.
Recall in Haskell a monad consists of following components:
A type constructor that defines for each ...
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Curry-Howard, void, and type checking in Haskell
I am trying to understand an example of theorem proving via type checking in Haskell given here. The example is as follows.
Using the Curry-Howard isomorphism, construct an inhabitant of the type and ...
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Can we think of a non-symmetric product type in Haskell?
Meta note: I asked this question here a while ago. It got an answer:
type a /\!! b = (a, ((b -> Void) -> Void))
Unfortunately, I do not reckon it to be ...
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For every imperative function, is there a functional counterpart with identical performance or even instructions?
Currently, I haven't learned about a functional language that can achieve the same performance as C/C++. And I have learned that some languages that favor functional programming to imperative ...
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Functor in category theory: The free theorem for fmap
According to nLab article:
https://ncatlab.org/nlab/show/functor
Definition
External definition
A functor $F$ from a category $C$ to a category $D$ is a map sending each object $x \in C$ to an object ...
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Is my understanding of strictness correct in this proof of a `foldl` rule?
Exercise G in Chapter 6 of Richard Bird's Thinking Functionally with Haskell asks the reader to prove
foldl f e . concat = foldl (foldl f) e
given the rule
...
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How do type classes make ad-hoc polymorphism less ad hoc?
The title of the paper that introduced type classes is "How to make ad-hoc polymorphism less ad hoc".
It seems the type classes approach is being compared to how OOP does ad-hoc polymorphism....
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Lambda Calculus Conversion
How can I take a Haskell data type or function (eg fold, list, String, zip) and convert or translate it to a lambda calculus abstraction?
Example:
If sum computes a sum of all elements in a list, and
:...
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How to view Graph Reduction/Graph Representation for a Haskell program?
I know that under the hood, for a Haskell program, the GHC compiler uses graph reduction for optimization. Is there any way to view this graphical representation of the program? I haven't been able to ...
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Prove simple theorems in Haskell in automated way
I would like to prove in Haskell, whether in vanilla Haskell or using some libraries / tools, some simple theorems such as:
...
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Haskell type of lambda expressions
I'm new to Haskell and have some general questions.
Question 1:
The Haskell expression (\x -> \x -> x) is the same as the λ-term ...
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Is the identity functor a kind of free object?
My understanding of free objects is: Free functors, free applicatives, free monads, free monoids, &c, give you more structure "for free", i.e. in general, or for all some thing with less structure,...
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Haskell: difference behavior in ghci concerning ``polymorphic recursion''
I stumbled upon some question that puzzled me, maybe it's just a feature
(or simply because I am doing first ``Haskell-steps'' without studying the
manual too deeply, which I guess I should...
Anyway,...
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Is possible to construct a fixed set of typeclases as powerful as unconstrainde typeclasses?
We can construct a fixed set of combinators with a computational power equivalent to lambda calculus.
Can we do the same with typeclasses (ad-hoc polymorphism)?
For example, construct a finite set ...
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Functor laws and natural transformations in Haskell
As I've been struggling to get a deeper understanding of monads in Haskell, I started reading about functors and their counterparts in category theory. Keep in mind that I have no background in the ...
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Semantics for de Bruijn levels
There is an exceptionally simple way to embed simply typed lambda calculus with de Bruijn indices in a functional host language (discussed by Carette, Kiselyov & Shan, and by Kiselyov). The ...
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Predecessor function with recursive types
I am defining the type Nat of natural numbers a recursive sum type:
$$ Nat = \mu X. Unit \oplus X$$
Now, I have defined zero ...
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How does Bifunctor in Haskell work?
I was reading 'Category Theory for Programmers' by Bartosz Milewski and got really confused by the implementation of bimap in Bifunctor typeclass.
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A notion dual to a product type having a given type
Consider this class:
class Has record part where
extract :: record -> part
update :: (part -> part) -> record -> record
It captures the notion of ...
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Why does higher-order abstract syntax need an inverse to define catamorphisms?
In the introduction to the colorfully-named Boxes Go Bananas: Encoding Higher-Order Abstract Syntax with Parametric Polymorphism, Washburn and Weirich describe a problem in traditional formulations of ...
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How does the function to curry and uncurrying another function work?
The following is the code to curry or uncurry a function in Haskell:
...
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Inferring the type of (f .) in Haskell
If we have the following in Haskell:
f x y = x + y
:type f
f :: Num a => a -> a -> a
then GHC would report ...
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Is the IO monad technically incorrect?
On the haskell wiki there is the following example of conditional usage of the IO monad (see here).
...
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Data types a la carte -- over-engineered?
I'm working through Swierstra's 2008 paper. I'm up to Section 3 eval addExample, just before 'Automating injections'. And boy! does it need automating.
Is it just ...
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