小编Bra*_*cki的帖子

在过去的几个月里,我一直在学习Haskell,我遇到了Monoids的例子让我感到困惑.

鉴于这些定义:

data Tree a = Empty | Node a (Tree a) (Tree a) deriving (Show, Read, Eq) 

instance F.Foldable Tree where  
    foldMap f Empty = mempty  
    foldMap f (Node x l r) = F.foldMap f l `mappend`  
                             f x           `mappend`  
                             F.foldMap f r  

而这棵树:

testTree = Node 5  
        (Node 3  
            (Node 1 Empty Empty)  
            (Node 6 Empty Empty)  
        )  
        (Node 9  
            (Node 8 Empty Empty)  
            (Node 10 Empty Empty)  
        )  

如果我跑:

ghci> F.foldl (+) 0 testTree  
42  
ghci> F.foldl …

我今年夏天和空闲时间一起练习Go图像包.

package main

import (

    "os"
    "image"
    "image/png"
    "image/color"
    "log"
    "fmt"
    "reflect"

)

func main(){

    file , err := os.OpenFile("C:/Sources/go3x3.png", os.O_RDWR, os.FileMode(0777))
    if err != nil {
        log.Fatal(err)
    }

    img , err := png.Decode(file)
    if err != nil {
        log.Fatal(err)
    }


    img.At(0,0).RGBA()
    fmt.Println("type:", reflect.TypeOf(img))

    m := image.NewRGBA(image.Rect(0, 0, 640, 480))
    fmt.Println("type:", reflect.TypeOf(m))
    m.Set(5, 5, color.RGBA{255, 0, 0, 255})
    img.Set(0, 0, color.RGBA{136, 0, 21, 255})
}

这里的问题是当我用img.Set注释掉的时候运行它我得到了这个结果

type: *image.RGBA
type: *image.RGBA

但当它没有注释时,我得到一个错误说

img.Set undefined (type image.Image has no field …