Add an `Iff` helper (#172)

I ran into some situation where I want to conditionally render a node if
some variable is not nil and obviously I got a panic

```go
// package viewmodels
type SomePage struct {
	s *string
}

// package views
func SomePage (vm viewmodels.SomePage) g,Node {
	return Div(
		If(vm.s == nil, Text("s is nil"),
		If(vm.s !- nil, Text("s is " + vm.s), // this will panic when `s` is nil
	)
}
```

In this situation, go will interpret the code of the second `if`
regardless of the condition because the code itself is not in a
condition.

This PR introduces a new `Iff` helper that accepts a callback. The
callback content is only interpreted when it's called, making the code
safe:

```go
// package viewmodels
type SomePage struct {
	s *string
}

// package views
func SomePage (vm viewmodels.SomePage) g,Node {
	return Div(
		Iff(vm.s == nil, func () g.Node { return Text("s is nil") },
		Iff(vm.s !- nil, func () g.Node { return Text("s is " + vm.s) },
	)
}
```

I'm aware of the `Lazy` effort on the side, but I guess this is no a
breaking change and can still exist in addition to the `Lazy` effort.

Co-authored-by: Markus Wüstenberg <markus@maragu.dk>

1 files changed, 11 insertions, 0 deletions

diff --git a/gomponents.go b/gomponents.go
index 7b73a3b..ae7cadc 100644
--- a/gomponents.go
+++ b/gomponents.go
@@ -254,9 +254,20 @@ func Group(children []Node) Node {
 
 // If condition is true, return the given Node. Otherwise, return nil.
 // This helper function is good for inlining elements conditionally.
+// If your condition and node involve a nilable variable, use iff because
+// go will evaluate the node regardless of the condition.
 func If(condition bool, n Node) Node {
 	if condition {
 		return n
 	}
 	return nil
 }
+
+// Iff execute the function f if condition is true, otherwise return nil.
+// it is the preferred way to conditionally render a node if the node involves a nilable variable.
+func Iff(condition bool, f func() Node) Node {
+	if condition {
+		return f()
+	}
+	return nil
+}