Tying the knot is a technique in which you can create circular data structures in the absence of mutation by referencing a yet to be produced value.
Questions tagged [tying-the-knot]
28 questions
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votes
3 answers
Is it possible to do a search on a graph constructed with the tying-the-knot strategy?
The tying-the-knot strategy can be used to construct graphs such as, using a simple two-edged graph as an example:
data Node = Node Node Node
-- a - b
-- | |
-- c - d
square = a where
a = Node b c
b = Node a d
c = Node a d
d =…
MaiaVictor
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Debugging and understanding "tying the knot" in a monadic context
I'm trying to implement an interpreter for a programming language with lazy-binding in Haskell.
I'm using the tying-the-knot pattern to implement the evaluation of expressions. However I found it extremely hard to debug and to reason about. I spent…
Blue Nebula
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How can I avoid <> in Haskell?
The program below results in <> in GHC.
...Obviously. In hindsight.
It happens because walk is computing a fixed point, but there are multiple possible fixed points. When the list comprehension reaches the end of the graph-walk, it "asks" for…
Jason Orendorff
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Functional Pearl: Implementing trace in JavaScript
Ross Paterson: Arrows and Computation introduces the trace function (on page 11):
trace :: ((a, c) -> (b, c)) -> a -> b
trace f a = let (b, c) = f (a, c) in b
The trace function is useful for modularizing the magic feedback step in circular…
Aadit M Shah
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Level-order repminPrint
repmin problem is pretty well-known. We are given a data type for trees:
data Tree a = Leaf a | Fork (Tree a) a (Tree a) deriving Show
We need to write a function down (repmin) which would take a tree of numbers and replace all numbers in it by…
Zhiltsoff Igor
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Birecursively defining a doubly infinite list of lists
Context
I asked about patching a recursively-defined list the other day. I'm now trying to bring it up a level by operating on a 2D list instead (a list of lists).
I'll use Pascal's triangle as an example, like for example this beautiful…
JB.
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Tie-the-knot in 2 dimensions (was: tying the knot with a comonad)
Edit: The original question was "tying the knot with a comonad", but what really helped here is a two-dimensional knot tying with U2Graph from cirdec. Original question (until Anwser):
I want to tie the knot with data that originates from a…
Franky
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Self-reference in data structure – Checking for equality
In my initial attempt at creating a disjoint set data structure I created a Point data type with a parent pointer to another Point:
data Point a = Point
{ _value :: a
, _parent :: Point a
, _rank :: Int
}
To create a singleton set, a…
beta
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Mutually recursive evaluator in Haskell
Update: I've added an answer that describes my final solution (hint: the single Expr data type wasn't sufficient).
I'm writing an evaluator for a little expression language, but I'm stuck on the LetRec construct.
This is the language:
type Var =…
Tom Lokhorst
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vote
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How does repmin place values in the tree in Haskell?
I really like the repmin problem:
Write down repmin :: Tree Int -> Tree Int, which replaces all the numbers in the tree by their minimum in a single pass.
If I were writing something like this in python, I would go for passing values by their…
Zhiltsoff Igor
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Haskell: handling cyclic dependencies while tying the knot
While writing a programming language that will feature local type inference (i.e. it will be capable of inferring types with the exception of function parameters, like Scala), I've run into a problem with cyclic dependencies.
I perform…
SongWithoutWords
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How to perform Tying the Knot/define observable recursively using iteself in Rx.Net?
Sometimes the business logic seems to be able to naturally modeled by some recursive defined observables. Here is one example:
interface Demo {
IObservable userCommands;
IObservable> processes;
…
xiang0x48
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Non-exhaustive patterns in aux function for tying the knot
I am trying to write a function in Haskell that takes a table and pads up cells of each column according to the maximum size of a string in that column. The way I am doing this is by using the technique - tying the knot. Here is the function that I…
Iguana
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