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--- becki:linux:golang [2011-02-16 13:52]
becki
+++ becki:linux:golang [2018-02-26 11:24] (aktuell)
becki
@@ Zeile 1: / Zeile 1: @@
-====== Go Programming Language Tips ======
+====== Go Programming Language Tips ==
 ===== Setup ==
@@ Zeile 14: / Zeile 14: @@
 #!/bin/sh
 export PATH="$PATH:/usr/local/go/bin"
+export GOROOT=/usr/local/go
 </code>
 Go Syntax Highlighting for your editor at http://go-lang.cat-v.org/text-editors/
+Go Syntax Highlighting for your dokuwiki at http://rosettacode.org/wiki/User:MizardX/GeSHi_Go.php (Doesn't work with 2009-12-25c "Lemming")
+Updating Go is described [[golang>doc/install.html?h=weekly+release#releases|here]]
+==== ARM5 (Sheevaplug) specific ==
+  * Current release version (release.r57.1 / 8295:95d2ce135523) does not work
+  * Current weekly version (weekly.2011-06-02 / 8624:3418f22c39eb) works
+  * ''export GOARM=5'' in ''~/.profile'' !
+  * export GOHOSTARCH=arm, export GOHOSTOS=linux, export GOARCH=arm, export GOOS=linux may be necessary too
+==== Build Crosscompiler on x86 for ARM5 ==
+Export the following variables before running ''src/all.bash'':
+<code bash>
+export GOROOT=$(pwd)
+export GOHOSTARCH=386
+export GOHOSTOS=linux
+export GOARCH=arm
+export GOOS=linux
+export GOARM=5
+</code>
 ===== Hello World ==
@@ Zeile 25: / Zeile 51: @@
 func main() {
-    fmt.Printf("Hallo Süße!\n")
+    fmt.Printf("Hallo  Süße!\n")
 }
 </code>
-Build & run: ''8g hello.go && 8l -o hello hello.8 && ./hello''
+Build & run:
+x86: ''8g hello.go && 8l -o hello hello.8 && ./hello''
+arm: ''5g hello.go && 5l -o hello hello.5 && ./hello''
 For ''Printf'' see [[golang>pkg/fmt/]]
+==== Command Line Arguments ==
+Use the ''os.Args'' slice. [[golang>doc/go_tutorial.html|Source]]
+==== Exit Code ==
+Use ''os.Exit()''.
+''return'' in ''main()'' only works without any argument, which results in ''0'' as exit code (tested).
+==== Standard Streams ==
+FIXME
 ===== Variable Declaration ==
 <code go>
 package main
 import "fmt"
 func main() {
     var i,j int                         // 2 ints, autoinitialized to zero value
     fmt.Printf("%d %d\n", i, j)         // 0 0
     var k,l,m,n= true, 0, 2.6, "Hello"  // missing types default to:
-    fmt.Printf("%T %T %T %T\n", k,l,m,n)// -> bool int float64 string
+    fmt.Printf("%T %T %T %T\n",k,l,m,n) // -> bool int float64 string
-    fmt.Printf("%v %v %v %v\n", k,l,m,n)// -> true 0 2.6 Hello
+    fmt.Printf("%v %v %v %v\n",k,l,m,n) // -> true 0 2.6 Hello
     fmt.Println(k,l,m,n)                // -> true 0 2.6 Hello
     o,p:= false, "World"                // short form: no type but initializers:
-    fmt.Printf("%T %T\n", o,p)          // -> bool string
+    fmt.Printf("%T %T\n", o, p)         // -> bool string
     fmt.Println(o,p)                    // -> false World
 }
@@ Zeile 58: / Zeile 101: @@
 <code go>
+a := [...]int{0, 1, 2, 3}           // an array created literally
+a := [4]int                         // create a zeroed array (?)
+</code>
+<code go>
 package main
 import "fmt"
 func main() {
-    //var ia= [...]int{47,11}       // Create an array literal
+    //var ia= [...]int{47, 11}      // Create an array literal
     ia:= [...]int{47,11}            // Create an array literal - short form
@@ Zeile 88: / Zeile 136: @@
 </code>
-  * The length of the array is part of its type and cannot grow
+<note tip>
-  * Arrays are real types, are copied by value, pointers to array are possible
+  * The //length// of the array is part of its type and cannot grow
+  * Arrays are real types
+  * Arrays are copied by value :!:
+  * A Pointer to an array is possible (unlike in C where the pointer represents the array)
+</note>
 ===== Slices ==
 <code go>
+s := []int{0, 1, 2, 3}                  // a slice created literally
+s := make([]int, 4)                     // create a zerored slice
+len(s) // get number of items in slice
+cap(s) // get actual slice capacity
+</code>
+<code go>
 package main
 import "fmt"
 func main() {
-    a:= [...]int{0,1,2,3}               // an array to play with
+    a:= [...]int{0, 1, 2, 3}            // an array to play with
     sa:= a[:]                           // Create slice from whole array
-    sb:= a[:2]                          // Create slice from last part
+    sb:= a[:2]                          // Create slice from first part
-    sc:= a[2:]                          // Create slice from first part
+    sc:= a[2:]                          // Create slice from last part
     fmt.Println(sa, len(sa), cap(sa))   // -> [0 1 2 3] 4 4
     fmt.Println(sb, len(sb), cap(sb))   // -> [0 1] 2 4(!)
@@ Zeile 127: / Zeile 188: @@
 }
 </code>
+<note tip>
+  * Slices are copied //by value// but the internal arrays are copied //by reference// :!:
+  * Slices have a length (number of items) and a capacity (length of underlying array(?))
+</note>
+<note important>Appending to a Slice results in a new slice. The new slice may point to a different array than the original slice.</note>
 [[http://blog.golang.org/2011/01/go-slices-usage-and-internals.html|more]]
+===== Objects ==
+==== Copying Objects ==
+<code go>
+type Point struct {
+    x, y int
+}
+func main() {
+    p:= Point{}
+    fmt.Println(p)                      // -> {0 0}
+    c:= p
+    p.y= 1
+    c.x= 2
+    fmt.Println(p, c)                   // -> {0 1} {2 0}
+}
+</code>
+<note tip>Objects are copied **by value**</note>
+==== Methods ==
+<code go>
+/** A method which has a copy of its object as receiver: */
+func (pt Point) SetWithVal(x, y int) {
+    pt.x= x
+    pt.y= y
+}
+/** A method which has a pointer to its object as receiver: */
+func (pt *Point) Set(x, y int) {
+    pt.x= x
+    pt.y= y
+}
+func main() {
+    p:= Point{}
+    /* invoking both methods on an object: */
+    ov:= p
+    ov.SetWithVal(1, 2)                 // -> SetWithVal operates only on copy!:
+    fmt.Println(ov)                     // -> {0 0}
+    ov.Set(3, 4)                        // -> Set works as expected:
+    fmt.Println(ov)                     // -> {3 4}
+    /* invoking both methods on pointer to object: */
+    op:= &p
+    op.SetWithVal(5, 6)                 // -> SetWithVal operates only on copy!:
+    fmt.Println(op)                     // -> &{0 0}
+    op.Set(7, 8)                        // -> Set works as expected:
+    fmt.Println(op)                     // -> &{7 8}
+    /* As expected, canges to the pointer change also the object pointed to: */
+    fmt.Println(p)                      // -> {7 8}
+}
+</code>
+<note tip>
+  * In order to really work on the object, the receiver of the method must be a //pointer// to the object, otherwise the method operates ony on an (anonymous) copy.
+  * Invoking methods on //pointers to objects// has the same syntax and work the same as invoking the method directly on the object.
+</note>
+==== Interfaces ==
+<code go>
+type Point struct {
+    x, y int
+}
+func (pt Point) SetWithVal(x, y int) {
+    pt.x= x
+    pt.y= y
+}
+func (pt *Point) Set(x, y int) {
+    pt.x= x
+    pt.y= y
+}
+type SetWithVal_i interface {
+    SetWithVal(x, y int)
+}
+type Set_i interface {
+    Set(x, y int)
+}
+func main() {
+    var isp Set_i
+    var isv SetWithVal_i
+    fmt.Printf("%T %T\n", isp, isv)     // <nil> <nil>
+    o:= Point{}
+    isv= o                              // isv is an independent COPY of o:
+    fmt.Printf("%T\n", isv)             // -> main.Point
+    o.x= 9                              // but the copy
+    isv.SetWithVal(1, 1)                // can't be modified via isv:
+    fmt.Println(o, isv)                 // -> {9 0} {0 0}
+    o= Point{}                          // reset to {0 0}
+    isv= &o                             // isv now is a POINTER(!) to o:
+    fmt.Printf("%T\n", isv)             // -> *main.Point
+    o.x= 9                              // updates to object are seen by isv
+    isv.SetWithVal(2, 2)                // but object can't be modified via isv:
+    fmt.Println(o, isv)                 // -> {9 0} &{9 0}
+    o= Point{}                          // reset to {0 0}
+    //isp= o                            // Err! Set_i.Set needs pointer receiver
+    isp= &o                             // isp now points to o:
+    fmt.Printf("%T\n", isp)             // -> *main.Point
+    isp.Set(3, 3)                       // object can be modified via interface
+    o.y= 4                              // and vice versa:
+    fmt.Println(o, isp)                 // -> {3 4} &{3 4}
+}
+</code>
+<note tip>
+An Interface can store any value that implemts it. This can be a value //or// a pointer to a value.
+The only way to directly operate on an object via an interface is to
+  * Implement the methods of the interface with an object //pointer// as receiver
+  * Instantiate the interface with the //adress// of the object
+</note>
+<note important>Some :?: library functions which return an interface in reality return a pointer to an implementation of the interface (see e.g. [[golang>pkg/net/#Listener.Listen|net.Listen]])</note>
+===== Handling Errors ==
+Defer, Panic, Recover: http://blog.golang.org/2010_08_01_archive.html ⇒ The convention in the Go libraries is that even when a package uses panic internally, its external API still returns explicit ''os.Error'' values.
+[[golang>pkg/os/#Error|os.Error]] is the same interface as [[golang>pkg/fmt/#Stringer|fmt.Stringer]], i.e. it has a method called ''String()'' wich returns a ''string''. Thus an instance of os.Error can always be passed to the functions in ''fmt'' and ''log'' directly, without explicitely calling the ''String()'' method. E.g:
+<code go>
+if err != nil {
+    log.Panic(err)
+}
+</code>
+FIXME See [[:becki:my:linux:exception_handling_in_c]]
+===== Unsorted Things ==
+  * Since [[golang>doc/go_tutorial.html#tmp_94|strings are immutable values]] I guess only references are passed around if you pass the type ''string''. Thus it probably does not make much sense to use pointers to strings.
+  * Seems to be convention that a function returns (among others) ''os.Error == nil'' when it succeded (tested)
+  * ''if'' and ''switch'' accept an initialization statement, see [[golang>doc/effective_go.html#if]]
+  * Number <=> String conversion is done with [[golang>pkg/strconv/]]
+  * [[http://www.syntax-k.de/projekte/go-review|Interesting Go overview]]
+===== Todo ==
+  * Where is variable argument list for functions?
+  * Where is ''basename''?
+  * Type Conversions look like function calls, see [[golang>doc/go_spec.html#Conversions]]
+  * Check type assertion, eg. ''s, ok := v.(Stringer)'', see Tutorial

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