Haskell docs解释Catastrophic failure (Exception from HRESULT: 0x8000FFFF (E_UNEXPECTED))函数：
在执行生成的IO操作时，强制将其参数计算为弱头正常形式。
CustomMapRenderer为什么using PROJECT;
using PROJECT.UWP;
using System.Collections.Generic;
using Windows.Devices.Geolocation;
using Windows.UI.Xaml.Controls.Maps;
using Xamarin.Forms.Maps;
using Xamarin.Forms.Maps.UWP;
using Xamarin.Forms.Platform.UWP;
[assembly: ExportRenderer(typeof(CustomMap), typeof(CustomMapRenderer))]
namespace PROJECT.UWP
{
public class CustomMapRenderer : MapRenderer
{
MapControl nativeMap;
CustomMap formsMap;
protected override void OnElementChanged(ElementChangedEventArgs<Map> e)
{
base.OnElementChanged(e);
if (e.OldElement != null)
{
nativeMap = Control as MapControl;
}
if (e.NewElement != null)
{
formsMap = (CustomMap)e.NewElement;
nativeMap = Control as MapControl;
UpdatePolyLine();
}
}
protected override void OnElementPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
base.OnElementPropertyChanged(sender, e);
if (this.Element == null || this.Control == null)
return;
if (e.PropertyName == CustomMap.RouteCoordinatesProperty.PropertyName)
{
UpdatePolyLine();
}
}
private void UpdatePolyLine()
{
if (formsMap != null && formsMap.RouteCoordinates.Count > 0)
{
List<BasicGeoposition> coordinates = new List<BasicGeoposition>();
foreach (var position in formsMap.RouteCoordinates)
{
coordinates.Add(new BasicGeoposition() { Latitude = position.Latitude, Longitude = position.Longitude });
}
Geopath path = new Geopath(coordinates);
MapPolyline polyline = new MapPolyline();
polyline.StrokeColor = Windows.UI.Color.FromArgb(128, 255, 0, 0);
polyline.StrokeThickness = 5;
polyline.Path = path;
nativeMap.MapElements.Add(polyline);
}
}
}
}
不是弱头范式，即evaluate不等于Prelude Control.Exception> let xs = [1..100] :: [Int] Prelude Control.Exception> :sprint xs
xs = _
Prelude Control.Exception> let ys = evaluate xs
Prelude Control.Exception> :t ys
ys :: IO [Int]
Prelude Control.Exception> ys
[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,6Prelu2,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100]
Prelude Control.Exception> :sprint xs
xs = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,
24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,
46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,
68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,
90,91,92,93,94,95,96,97,98,99,100]
Prelude Control.Exception> :sprint ys
ys = _
？
答案 0 :(得分：10)
您的ys值的类型为IO [Int]。现在，IO是一种抽象类型，在您的情况下可以被认为是RealWorld -> ([Int], RealWorld)。现在这个IO值已经处于弱头正常状态。这就是为什么当你对_进行sprint时将其视为ys。
为什么
:sprint ys不是弱头范式，即_ : _不等于ys？
_ : _外部字词不能为IO [Int]，因为它不是列表，而是{{1}}类型的值。
答案 1 :(得分：9)
除了Sibi所说的内容之外，还有一种方法可以看出evaluate实际上做了文档所说的内容：
GHCi> let xs = [1..100] :: [Int]
GHCi> :sprint xs
xs = _
GHCi> let a = evaluate xs >> return ()
GHCi> a
GHCi> :sprint xs
xs = 1 : _