data.maybe | Migrated to https : //github.com/origamitower/folktale | Functional Programming library
kandi X-RAY | data.maybe Summary
kandi X-RAY | data.maybe Summary
Migrated to https://github.com/origamitower/folktale
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data.maybe Key Features
data.maybe Examples and Code Snippets
Lines of Code : 116License : Strong Copyleft (CC BY-SA 4.0)
Copymodule Main where
import Prelude
import Data.Foldable (oneOf)
import Data.Generic.Rep (class Generic)
import Data.Generic.Rep.Show (genericShow)
import Data.Maybe (Maybe(..), isNothing)
import Debug.Trace (spy)
import Effect (Effect)
impoimport Data.Aeson
import Data.Maybe
import qualified Data.ByteString.Lazy as B
getMoves :: IO Value
getMoves = do
mv <- decode <$> B.readFile "moves.json"
case mv of
Nothing -> error "invalid JSON"
Just v -> retimport Control.Monad.State
import qualified Data.Map as M
import Data.Maybe
data Var = Var Int
deriving (Show, Eq, Ord)
data Env = Env {freshVar :: Int, vars :: M.Map Var Int}
deriving Show
type Imperative a = State Env a
import Data.Maybe (isJust, listToMaybe)
-- Find linear combinations of positive integers
solve :: Integer -> [Integer] -> Maybe [Integer]
-- If we've made it to the end with zero left, good!
solve 0 [] = Just []
-- Otherwise, thdata Foo = Bar Int | Baz String
data True = TrueThing
{-# LANGUAGE DuplicateRecordFields #-}
data CreateWorkspace = CreateWorkspace
{ commandId :: UUID
, workspaceId ::UUID
} deriving{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE DeriveGeneric #-}
module Main where
--import Text.Groom
import Control.Monad.Trans.Resource
imp{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE OverloadedStrings #-}
module Main where
import Conduit ((.|))
import qualified Conduit as C
import Control.Concurrent
import # Serving an API
Enough chit-chat about type-level combinators and representing an API as a
type. Can we have a webservice already?
## A first example
Equipped with some basic knowledge about the way we represent APIs, let's now write oghci> :set -XOverloadedStrings
ghci> JSON.decode "{\"children\":[],\"value\":1}" :: Maybe (RoseTree Int)
Just (RoseTree 1 [])
ghci> JSON.decode "[]"
Just ()
{-# LANGUAGE Overloaded{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGCommunity Discussions
Trending Discussions on data.maybe
QUESTION
Consider the following PureScript code, a slightly modified version of an example from the handbook:
...ANSWER
Answered 2022-Mar-31 at 13:30Array is not a type class, but a type.
You accidentally put a fat arrow after Array a instead of a thin arrow, making it look like you were declaring a value of type Int with a constraint Array a rather than a function that takes an Array a as a parameter and returns an Int. This is how it should look:
QUESTION
I have a list with this type: [(a, [a])].
For example:
ANSWER
Answered 2022-Feb-17 at 11:10You can make a recursive function that filters the rest of the list with the items in the second item of the 2-tuples, so:
QUESTION
here is a puzzle that I keep on bumping into and that, I believe, no previous SO question has been addressing: How can I best use the lens library to set or get values within a State monad managing a nested data structure that involves Maps when I know for a fact that certain keys are present in the maps involved?
ANSWER
Answered 2022-Feb-09 at 11:43If you are sure that the key is present then you can use fromJust to turn the Maybe User into a User:
QUESTION
at the moment I'm writing a small program which runs a couple SQLServer queries. To make the queries type-save I created a module "SQLQuery" which lets you design queries in a save way.
To execute the queries they have to be of type Query thus I did the following...
...ANSWER
Answered 2022-Jan-29 at 16:49The Haskell ODBC library is designed to do string escaping. What seems to be happening is that toSql "String" wraps up the contents of the string as an escaped SQL string, not an unescaped query.
If you use fromString myQuery instead of toSQL $ T.pack myQuery it should work.
QUESTION
I am writing a DNA a translator using Haskell (bytestrings in particular). I have the following code:
...ANSWER
Answered 2021-Dec-14 at 19:02compStrand is a very strange function. Why does it take a Maybe ByteString instead of an actual ByteString when you only pass it a Just? Why does it do this wacky foldr ((<>) . compPairs) Nothing thing which is just a very hard-to-read reimplementation of (>>= compPairs)?
main is not much clearer: If your input is GATC strings, why are you splitting on the first byte equal to 10? Why doesn't it use this xtractDNA function you wrote for it?
Resolving these questions will lead to a simpler function that you can debug more easily on your own. But I will note that it seems like your compStrand function operates only on singleton strings, and yet you seem to be passing it a string of unknown size.
QUESTION
Suppose I have some very simple code:
...ANSWER
Answered 2021-Nov-13 at 18:45You can use a pattern guard:
QUESTION
My two submissions for a programming problem differ in just one expression (where anchors is a nonempty list and (getIntegrals n) is a state monad):
Submission 1. replicateM (length anchors - 1) (getIntegrals n)
Submission 2. sequenceA $ const (getIntegrals n) <$> tail anchors
The two expressions' equivalence should be easy to see at compile time itself, I guess. And yet, comparatively the sequenceA one is slower, and more importantly, takes up >10x memory:
(with "Memory limit exceeded on test 4" error for the second entry, so it might be even worse).
Why is it so?
It is becoming quite hard to predict which optimizations are automatic and which are not!
EDIT: As suggested, pasting Submission 1 code below. In this interactive problem, the 'server' has a hidden tree of size n. Our code's job is to find out that tree, with minimal number of queries of the form ? k. Loosely speaking, the server's response to ? k is the row corresponding to node k in the adjacency distance matrix of the tree. Our choices of k are: initially 1, and then a bunch of nodes obtained from getAnchors.
ANSWER
Answered 2021-Nov-09 at 22:52The problem here is related to inlining. I do not understand it completly, but here is what I understand.
InliningFirst we find that copy&pasting the definition of replicateM into the Submission 1 yields the same bad performance as Submission 2 (submission). However if we replace the INLINABLE pragma of replicateM with a NOINLINE pragma things work again (submission).
The INLINABLE pragma on replicateM is different from an INLINE pragma, the latter leading to more inlining than the former. Specifically here if we define replicateM in the same file Haskells heuristic for inlining decides to inline, but with replicateM from base it decides against inlining in this case even in the presence of the INLINABLE pragma.
sequenceA and traverse on the other hand both have INLINE pragmas leading to inlining. Taking a hint from the above experiment we can define a non-inlinable sequenceA and indead this makes Solution 2 work (submission).
QUESTION
Consider this trivial algorithm of prime-decomposition of an integer n: Let d' be the divisor of n last found. Initially, set d'=1. Find the smallest divisor d>d' of n, and find the maximal value e such that de divides n. Append de to the answer and repeat the procedure on n/de. Finally, stop when n becomes 1. For simplicity, let's ignore mathematical optimizations, like stop at sqrt n etc.
I have implemented it in two ways. The first one generates a list of division "attempts", and then groups the successful ones by divisors. For example, for n=20, we first generate [(2,20),(2,10),(2,5),(3,5),(4,5),(5,5),(5,1)], which we then transform to the desired [(2,2),(5,1)] using group and other library functions.
The second implementation is an explicit recursion which keeps track of the exponent e along the way, appends de to the answer once the maximal e is reached, proceeds to finding the "next" d, and so on.
Question 1: Why does the first implementation run way slower than the second, despite the following:
- Both the implementations execute
div, the core step of the algorithm, roughly the same number of times. - Lazy evaluation (and fusion?) has the effect that the long list illustrated above never has to be materialized in the first place. As you can see in the code below,
divTrials n, the list I am talking about, is transformed by a chain of higher order functions. In that, I think that the partmap (\xs-> (head xs,length xs)) ... groupshould tell the compiler that the list is just intermediate:
ANSWER
Answered 2021-Oct-21 at 15:24Just to establish a baseline, I ran your program on a slightly larger starting number (so that time didn't print out 0.00s). I chose n = 2938722345623 for no particular reason. Here's the timings before starting to tweak things:
ans1: indistinguishable from infinity (I finished writing this entire answer and it was still running, about 26 minutes in total)
ans2: 2.78s
The first thing to try is to tweak this line:
QUESTION
What is the syntax to include a module in a Haskell source code file? I am writing
...ANSWER
Answered 2021-Oct-05 at 06:39The reason you get this error is because you imported the module at the wrong place in the file. The import statements should be written at the top of the file, immediately after a module statement (if you use one). You thus should not import a module after you defined one (or more) types, functions, etc.
A file thus looks like:
QUESTION
In Maguire's Thinking with Types, p. 29, there's an example of how to use a promoted data constructor as a phantom parameter. Here's a module that I wrote based on the example in the book.
...ANSWER
Answered 2021-Sep-17 at 21:16With the original code, you can write:
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