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Functions on the left-hand side of a for-range statement
For statements with range clause … Function calls on the left are evaluated once per iteration. This small sentence is easy to overlook. What does it mean? It means if you have any function calls on the left hand side of the for range statement, they are executed per iteration. What would that even look like? It’s probably a rare occurrence, but here’s an example in the playground: func main() { for _, m()["foo"] = range []string{"foo", "bar", "baz"} { } fmt.
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Evaluation of range expressions
For statements with range clause … The range expression x is evaluated once before beginning the loop, with one exception: if at most one iteration variable is present and len(x) is constant, the range expression is not evaluated. The first part of this seems pretty self-evident, as it follows the same pattern as a for statement with a for clause: The expression needs to be evaluated once before the loop executes.
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For statements with range clause
Yesterday we saw some high-level examples of range statements. Let’s start into some of the details now. For statements with range clause … For each entry it assigns iteration values to corresponding iteration variables if present and then executes the block. RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression . The formal definition there is pretty straight forward. But there are a number of nuances, which we’ll cover over the next few days.
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Rangeable values
For statements with range clause A “for” statement with a “range” clause iterates through all entries of an array, slice, string or map, values received on a channel, or integer values from zero to an upper limit [Go 1.22]. Wow, that’s a long list. And it’s about to get longer! Go 1.23 is scheduled to add iterator functions to that list, but we’ll talk about that when the time comes (in August).
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Final notes on for loop variable scope
For statements with for clause … The variable used by the first iteration is declared by the init statement. The variable used by each subsequent iteration is declared implicitly before executing the post statement and initialized to the value of the previous iteration’s variable at that moment. So let’s break this down. In fact, the pseudo code I showed you during my earlier breakdown really more closely demonstrated the behavior prior to Go 1.
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Unique loop variables and the Go 1.22 fix
If you haven't joined me for a livestream in the past, consider [joining today](https://youtube.com/live/BYOsoZ5ywx8)! I'll be doing some refactoring, and adding new features to [Kivik](https://kivik.io/), with TDD. For statements with for clause … Each iteration has its own separate declared variable (or variables) [Go 1.22]. This seemingly simple sentence carries a lot of weight. So that’s what we’ll be talking about today. Prior to Go 1.22 (which only came out earlier this year!
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Omitting for clause elements
For statements with for clause … Any element of the ForClause may be empty but the semicolons are required unless there is only a condition. If the condition is absent, it is equivalent to the boolean value true. for cond { S() } is the same as for ; cond ; { S() } for { S() } is the same as for true { S() } So this goes to show that the single condition form of the for loop is really just a special case of the “for clause” for loop, with the init and post statements omitted.
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2 min read
For statements with for clause
Today we’ll look at the second, of three, types of for statements: Those with a for clause. For statements with for clause A “for” statement with a ForClause is also controlled by its condition, but additionally it may specify an init and a post statement, such as an assignment, an increment or decrement statement. The init statement may be a short variable declaration, but the post statement must not. ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
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For statements
Go is known as a simple language, with straightforward concepts. For loops may be one of rare exceptions to that straightforward character, with three different forms: For statements A “for” statement specifies repeated execution of a block. There are three forms: The iteration may be controlled by a single condition, a “for” clause, or a “range” clause. ForStmt = "for" [ Condition | ForClause | RangeClause ] Block . Condition = Expression .
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Type switches conclusion
Two final items with regard to type switches, before we move on to the next topic: Type switches … The type switch guard may be preceded by a simple statement, which executes before the guard is evaluated. This is pretty straight forward. It works just like the simple statements in if statements and normal switch statements. A simple statement can be used both with or without a temporary variable. In other words, both of these are valid:
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Type switches with generics
More live coding today! Join me! And bring your questions. Type switches … A type parameter or a generic type may be used as a type in a case. If upon instantiation that type turns out to duplicate another entry in the switch, the first matching case is chosen. func f[P any](x any) int { switch x.(type) { case P: return 0 case string: return 1 case []P: return 2 case []byte: return 3 default: return 4 } } var v1 = f[string]("foo") // v1 == 0 var v2 = f[byte]([]byte{}) // v2 == 2 The example included is nice, because it shows two examples of the type parameter P being used: Both the first case (case P) and third (case []P) use the type parameter.