Friday, January 11, 2019

Understanding Scope in Go

As per my New Year's resolution, I've been learning to program in Go and reading The Go Programming Language.   On page 141 of the book, there are a couple of code examples to explain scoping rules and how variables are bound to anonymous functions.  This short post is just me making sure I grok what's in the book and sharing in case its helpful to anyone else.

Consider this snippet:

mynums is an array of functions.  The initialization clause of the for loop creates an implicit lexical block that encloses the variable i.  This variable i is in scope through every iteration of the loop.  That means that the same i is updated in each iteration and bound to the anonymous function that is appended to mynums by append(mynums, func () { fmt.Println(i) }).  When the first entry is appended to mynums, the value of i is 1, but the value is NOT what is bound to the function.  The variable itself (the location) is bound.  So every iteration appends a function referencing the same variable.  At the end, we have ten functions in the array and all of them are bound to the same variable which has one value: 11.  If we compile and run the program, it prints out "11", ten times.

This is analogous to the second example on page 141 of the book.  Now, consider this code sample:

Here, we add the line j := i and use j for the anonymous function func () { fmt.Println(j)})j is declared inside the body of the for loop.  The scope of j is a particular instance of the for loop.  Over ten iteration of the loop, we will declare ten separate instances of j.  That means that the first j bound to the anonymous function appended to mynums will have the value 1 and that value will not change.  The second iteration will bind a different j that has the value 2.  When we execute the various f() at the end, we will get the output we expect, the numbers 1 to 10.

If you're reading the book, which I highly recommend, you may also want to check out the discussion of scope on pages 46-47.

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Understanding Scope in Go

As per my New Year's resolution, I've been learning to program in Go and reading  The Go Programming Language .   On page 141 of the...