purescript | 易学教程

Put adjacent elements in List into Tuples

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问题 Given a List of elements: xs = [a, b, c, d, ... z] where a, b, c etc are placeholders for arbitrary values. I want to implement a function adjacents :: [a] -> [(a, a)] that produces adjacentValues = [(a, b), (b, c), (c, d), ... (y, z)] In Haskell, a recursive definition is reasonably concise: adjacents :: [a] -> [(a, a)] adjacents (x:xs) = (x, head xs) : adjacents xs adjacents [] = [] Purescript is a little more verbose: adjacents :: forall a. List a -> List (Tuple a a) adjacents list = case

How do you install a specific package version with Spago?

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问题 With tools like npm we can install a specific version npm install foo@1.2.0 How do you install a specific version using spago install ? 回答1: Firstly, that's not what spago install does. Instead of "adding a package to your project", spago install downloads all packages that are currently referenced in your spago.dhall file. Secondly, the idea with Spago is that you don't choose a specific package version. Instead what you choose is a "snapshot", which is a collection of certain versions of

Similar record types in a list/array in purescript

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问题 Is there any way to do something like first = {x:0} second = {x:1,y:1} both = [first, second] such that both is inferred as {x::Int | r} or something like that? I've tried a few things: [{x:3}] :: Array(forall r. {x::Int|r}) -- nope test = Nil :: List(forall r. {x::Int|r}) {x:1} : test -- nope type X r = {x::Int | r} test = Nil :: List(X) -- nope test = Nil :: List(X()) {x:1} : test {x:1, y:1} : test -- nope Everything I can think of seems to tell me that combining records like this into a

2020排行榜！Realworld前端框架的比较

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全文共1500字，预计学习时长6分钟图源：unsplash 过去的三年我们都探讨了这个话题，那么今年的情况会是怎样呢？首先声明，此文并不是关于未来前端选择的比较，而是从三个方面（性能，大小，相似应用下的代码行数）来进行小范围的简单比较。读者需要注意: · 本文是在比较Realworld软件——而不是正在研发中的软件，这些软件通常缺乏足够的知识和想法，因此难以实现。 · 由专家撰写或评审过——理想情况下，该技术领域的专家会评估此项目。 · 以某种方式标准化—— 一个符合特定规则的项目存在一种规范，提供后端API，静态标记和样式。正在比较哪些库/框架？撰写此文时，Realworld存储库中有24种conduit实现As，它们之间的从属地位并不重要，唯一的判定标准是看它是否出现在RealWorld repo page上。关注的是什么指标？性能—此应用程序需要多长时间才能显示内容并可用？大小—该应用程序有多大？我们将只比较已编译的JavaScript文件的大小。HTML和CSS对所有变体都是通用的，并且是从CDN（内容交付网络）下载的。所有技术都可以编译或转换为JavaScript，因此仅调整该文件的大小。代码行数—需要多少行代码才能基于规范创建RealWorld应用程序？某些应用程序很麻烦，但应该不会产生重大影响。我们量化的唯一文件夹是每个应用程序中的src /

Error running purescript example

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问题 I'm trying to learn purescript starting with the simple "Hello World" from "Purescript by Example". Issuing a "pulp run" throws this error: * Building project in /home/peter/devel/purescript/my-project Error found: at bower_components/purescript-psci-support/src/PSCI/Support.purs line 21, column 1 - line 21, column 1 Unable to parse module: unexpected "else" expecting declaration or end of input purs is of version 0.11.7, Pulp is version 12.2.0. What am I doing wrong? Thanks for your help.

“undefined value, reference not allowed” workaround

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问题 I'm looking for some clarification on the compiler error message The value of xyz is undefined here, so reference is not allowed. , together with the do-notation. I did not manage to generalize the example enough, all I can give is the concrete example where I stumbled upon this behaviour. Sorry for that. Using purescript-parsing, I want to write a parser which accepts nested multiline-comments. To simplify the example, each comment starts with ( , ends with ) and can contain either an a , or

How can I encode and enforce legal FSM state transitions with a type system?

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问题 Suppose I have a type Thing with a state property A | B | C , and legal state transitions are A->B, A->C, C->A . I could write: transitionToA :: Thing -> Maybe Thing which would return Nothing if Thing was in a state which cannot transition to A . But I'd like to define my type, and the transition functions in such a way that transitions can only be called on appropriate types. An option is to create separate types AThing BThing CThing but that doesn't seem maintainable in complex cases.

Purescript MouseEvent to get x y of mouse

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问题 I am new to purescript and was trying to make a 3d cube rotate on mouse events. But I cannot get x and y coordinates of mouse pointer on mouse move event. I am attaching my code below which have a event listener. Can somebody help me in getting x and y coordinates of mouse or can tell me better way to write event listener for mouse. node <- querySelector "#canvas" for_ node $ addEventListener "mousedown" $ void do modifyRef drag \d -> true xz <- getPageX log (show xz) x <- liftEff $ Window

Automatically focus input element after creation in purescript-halogen

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问题 I'm using purescript-halogen to build a spreadsheet-like table (similar to Handsontable). If you double-click a cell, an html input element is rendered as a child of the respective table cell (and no such element is rendered for all the other cells). This works really well with halogen, except that I don't know how to automatically set the focus to the newly created input element. I tried the autofocus attribute, but this only works for the first cell that is double-clicked. The JavaScript

Thinking Functionally. Building a New Array in Haskell / Purescript

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问题 I'm new to functional programming, and I've decided to build an app in Purescript. I've hit my first hurdle, and I'm not sure how to think about this conceptually. I'm not looking for code as much as a way to think functionally about this problem. I have a list of data. Specifically, something like [ {a :: String, b :: String, c :: String} ] I would like to create a list of Html (which is a purescript-halogen type) by using the record provided (with a list of the above types). So, I would