Make sure statements in pipelines are not allowed

[kubouch]

Statements shouldn't be allowed in pipelines to prevent errors like the following:

• let x = let y = 5
• return cmd1 | cmd2 (returns only cmd1)

Related: #37

[NotTheDr01ds]

A couple of questions …

• They should still be allowed as the last statement in a pipeline so that you can, for instance, return $in, right?
• What, technically, is a “statement” in Nushell?

[kubouch]

Statement means that it doesn't have a meaningful value that you can pipe into something. Like let or def, they don't have any value. return is a bit weird because it returns a value, but it's a control statement which returns a value from the containing block, but doesn't have a return value itself. You can't pipe return into anything. Other examples of statements are while or for -- they don't return any value. However, we could allow break $value which would turn while and for into expressions. Similarly, if is an expression because it returns a value, e.g., let x = if ($y > 2) { 3 } else { 4 }.

Some incomplete PEG grammar sketch to showcase the hierarchy:

Block      <- (Statement / Pipeline) (('\n' / ';') Statement / Pipeline)*
Statement  <- Return / Break / Let / Def ...
Pipeline   <- Expression ('|' Expression)*
Expression <- If / String / Int / MathOperation / CommandCall / ...
Def        <- 'def' Name '[' Params ']' (':' ReturnTypes)? Block

I'm kind of wrapping my head around it as I go, so I might be missing something, though.