purescript

问题 data Foo a = Foo a I can create an array of Exists https://github.com/purescript/purescript-exists [(mkExists (Foo 0)), (mkExists (Foo "x"))] How can I use type classes? I want to get ["0", "x"] getStrings :: Array (Exists Foo) -> Array String getStrings list = map (runExists get) list where get :: forall a. Show a => Foo a -> String get (Foo a) = show a No type class instance was found for Prelude.Show _0 The instance head contains unknown type variables. Consider adding a type annotation.

问题 data Foo a = Foo a I can create an array of Exists https://github.com/purescript/purescript-exists [(mkExists (Foo 0)), (mkExists (Foo "x"))] How can I use type classes? I want to get ["0", "x"] getStrings :: Array (Exists Foo) -> Array String getStrings list = map (runExists get) list where get :: forall a. Show a => Foo a -> String get (Foo a) = show a No type class instance was found for Prelude.Show _0 The instance head contains unknown type variables. Consider adding a type annotation.

问题 I am new to Purescript and I am trying to write a function that can take any record value and iterate over the fields and values and build a querystring. I am thinking something like: buildQueryString :: forall a. PropertyTraversible r => r -> String which I want to use like this: buildQueryString {name: "joe", age: 10} -- returns: "name=joe&age=10" Is there a way to write something like that in Purescript with existing idioms or do I have to create my own custom Type Class for this? 回答1:

问题 I am trying to use purescript-thermite to build an application using websockets. The idea is that the application connects to some server using websockets and live-updates the HTML page. However, I cannot find a way how to wire it into the thermite workflow. I have a spec that is made up of render and performAction . The render has access to the dispatch function. However, I need to start the websockets before rendering the element (I could probably put it e.g. into main ), but upon arrival

问题 In Haskell I can write a multi-line statement on a single line, like do {x<-[1,2];y<-[3,4];return (x,y)} but in Purescript even a single-statement do-statement with curly braces will not compile. Is there different syntax to achieve this? 回答1: No, PureScript has no syntax for doing this kind of thing. Aside from just not using do ;) [1, 2] >>= \x -> [3, 4] >>= \y -> pure (Tuple x y) 来源： https://stackoverflow.com/questions/46461251/write-multi-line-statement-on-single-line

问题 I'm looking for an idiomatic way to write a function List Char -> String in Purescript. This seems like a simple thing to do, but I'm new to Purescript and have been browsing documentation for a while now with no progress! Background information: I am porting a simple function from Haskell to Purescript generateId :: Int -> [Char] This generates a String of specified length. It was quite easy to convert the code to use List Char operations (where List is from Data.List in Purescript). In

问题 So, I am attempting to develop a function that expects a polymorphic function. I start with and create a class Foo , to represent the constrained type argument in the example function. module Test where class Foo a Along with a random type for this example... newtype Bar = Bar Int In order to pass Bar to anything that has a Foo constraint, it needs to have a typeclass, so I do so: instance fooBar :: Foo Bar Everything should be perfectly fine, however, this doesn't work, psc doesn't think the

问题 Error found: in module Test at /Users/arahael/dev/test/test.purs line 14, column 37 - line 14, column 59 Could not match constrained type (Spreadable a0) => Array a0 -> Int with type (Spreadable a1) => Array a1 -> Int while trying to match type (Spreadable a0) => Array a0 -> Int with type (Spreadable a1) => Array a1 -> Int while checking that expression mkFn2 get_biscuits has type Fn2 String (forall a. (Spreadable a) => Array a -> Int) Int in value declaration packetMaker where a1 is a rigid

问题 I am trying to do this data Foo a = Foo a data FooWrapper = FooWrapper (forall a. Foo a) foo = [FooWrapper (Foo 0), FooWrapper (Foo "")] But there is an error Could not match type Int with type a0 回答1: Existential types don't quite work the same in PureScript as they do in Haskell, so usually we use the purescript-exists library for this kind of thing. The equivalent using Exists would be: import Data.Exists (Exists(), mkExists) data Foo a = Foo a data FooWrapper = FooWrapper (Exists Foo) foo

问题 In a PureScript Halogen project, I would like to set the state to a random number, but how do I extract the value? The normal r <- randomInt 1 10 does not compile when it's inside the eval function. module Main where import Prelude import Control.Monad.Eff (Eff) import Control.Monad.Eff.Random (randomInt, RANDOM) import Halogen as H import Halogen.HTML.Events.Indexed as HE import Halogen.HTML.Indexed as HH import Halogen.Util (runHalogenAff, awaitBody) type State = { n::Int } initialState ::