improvements to vecty.If/vecty.MarkupIf

[cj123]

The current way of using vecty.If() and vecty.MarkupIf() is unsafe under certain circumstances that a normal if would prevent against.

For example, take a simple nil check in go:

var foo *Something

if foo != nil {
  // you can safely use foo as a *Something here.
} 

the same principle does not however apply in Vecty.

vecty.If(foo != nil, foo.Bar) // foo.Bar will cause a panic here if foo is nil

This is because foo.Bar is evaluated before the method call, regardless of whether foo is nil or not.

One way to avoid this would be to make the second parameter to vecty.If a function, e.g.

func If(cond bool, f func() vecty.ComponentOrHTML) vecty.MarkupOrChild {
	if cond {
		return f()
	}

	return nil
}

Doing this would allow for safer uses of vecty.If without requirement of extra checks elsewhere, as the func is only called if the condition is met.

e.g.

vecty.If(foo != nil, func() vecty.ComponentOrHTML {
    return elem.Div(
         vecty.Text(foo.Bar),
    )
})

I can see however how this is more verbose and perhaps not as flexible as vecty.If's current method signature. Just interested in opening a discussion about how this could be improved.