将多个信号传递给Signal.map抛出类型不匹配错误

Question

我正在尝试使用elm-0.15打印鼠标光标与窗口中心的距离。例如，将光标放置在窗口中心必须打印(0,0)。

我的代码是，

import Graphics.Element exposing (show)
import Mouse
import Signal
import Window

relativeMouse : (Int, Int) -> (Int, Int)
relativeMouse (ox, oy) (x,y) = (x - ox, y - oy)

center: (Int, Int) -> (Int, Int)
center (w,h) = (w/2, h/2)

main = Signal.map show <| relativeMouse (Signal.map center Window.dimensions Mouse.position)

elm-make basics.elm抛出至少4个单独的type-mismatch errors

如何将多个信号(window.dimensions，Mouse.position)传递给elm 0.15中的一个函数(例如，relativeMouse)？

Answer 1

注释是与您所拥有的内容不同的更改：

import Graphics.Element exposing (show)
import Mouse
import Signal
import Window

-- change type signature to match implementation
relativeMouse : (Int, Int) -> (Int, Int) -> (Int, Int)
relativeMouse (ox, oy) (x,y) = (x - ox, y - oy)

-- use // for integer division
center: (Int, Int) -> (Int, Int)
center (w, h) = (w // 2, h // 2)

-- 1) map center over Window.dimensions to get a signal of origin positions
-- 2) map2 relativeMouse over the signal of origin positions and
--    Mouse.position to get signal of relative mouse positions
-- 3) map show over the signal of relative mouse positions to display them
main = Signal.map show (Signal.map2 relativeMouse (Signal.map center Window.dimensions) Mouse.position)

要回答最后一个问题：Signal.map2是用来映射两个信号上的一个函数的。对应的映射一直存在到map5。

信号导入线也可以更改为

import Signal exposing ((<~), (~))

使用较短的信号映射语法，在这种情况下，主线将更改为

main = show <~ (relativeMouse <~ (center <~ Window.dimensions) ~ Mouse.position)