WTF Go

Resources

Can't Use "nil" to Initialize a Variable Without an Explicit Type

Fails:

package main

func main() {
    var x = nil //error

    _ = x
}
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Works:

package main

func main() {
    var x interface{} = nil

    _ = x
}
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Using "nil" Slices and Maps

It's OK to add items to a "nil" slice, but doing the same with a map will produce a runtime panic.

Fails:

package main

func main() {
    var m map[string]int
    m["one"] = 1 //error
}
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Works:

package main

func main() {
    var s []int
    s = append(s,1)
}
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Map Capacity

You can specify map capacity when created, but you can't use the cap() function on maps.

package main

import (
   "fmt"
)

func main() {
   a := make([]int, 99)
   fmt.Println(cap(a)) // 99

   b := make(map[string]int, 99)
   fmt.Println(len(b)) // 0
   fmt.Println(cap(b)) // error: ./prog.go:12:17: invalid argument b (type map[string]int) for cap
}
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String can't be nil

Fails:

package main

func main() {
    var x string = nil //error

    if x == nil { //error
        x = "default"
    }
}
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Works:

package main

func main() {
    var x string //defaults to "" (zero value)

    if x == "" {
        x = "default"
    }
}
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Array Func Args

If you are a C or C++ developer arrays for you are pointers. When you pass arrays to functions the functions reference the same memory location, so they can update the original data. Arrays in Go are values, so when you pass arrays to functions the functions get a copy of the original array data. This can be a problem if you are trying to update the array data.

Fails:

package main

import "fmt"

func main() {
    x := [3]int{1,2,3}

    func(arr [3]int) {
        arr[0] = 7
        fmt.Println(arr) //prints [7 2 3]
    }(x)

    fmt.Println(x) //prints [1 2 3] (not ok if you need [7 2 3])
}
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Works:

package main

import "fmt"

func main() {
    x := [3]int{1,2,3}

    func(arr *[3]int) {
        (*arr)[0] = 7
        fmt.Println(arr) //prints &[7 2 3]
    }(&x)

    fmt.Println(x) //prints [7 2 3]
}
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Another option is to use slices. Even though your function gets a copy of the slice variable it still references the original data.

package main

import "fmt"

func main() {
    x := []int{1,2,3}

    func(arr []int) {
        arr[0] = 7
        fmt.Println(arr) //prints [7 2 3]
    }(x)

    fmt.Println(x) //prints [7 2 3]
}
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Unexpected Values in Slice and Array "range" Clauses

Continue

Strings Are Immutable

Fails:

package main

import "fmt"

func main() {
    x := "text"
    x[0] = 'T'

    fmt.Println(x)
}
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package main

import "fmt"

func main() {
    x := "text"
    xbytes := []byte(x)
    xbytes[0] = 'T'

    fmt.Println(string(xbytes)) //prints Text
}
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