interp: aliases cannot contain pipes

[alexflint]

Running alias a='b | c' through interp produces this error:

`|` is not a valid word

This is because the interpreter processes aliases by parsing the expression on the right as a sequence of words here. There is a suggestive TODO just above those lines:

// TODO: parse any CallExpr perhaps, or even any Stmt

However, further down the line it seems that a slice of words is needed in order to call expand.Fields. I guess this is actually a pretty non-trivial change because parsing arbitrary CallExprs or Stmts within aliases will necessitate some AST surgery later on in order to replace the aliased word/token with the parsed CallEpr or Stmt.

Any advice on how to implement this?

[mvdan]

Indeed this is probably a non-trivial change. I put this off because I assumed not many people would care.

I would suggest that you check how Bash implements this. How does it do alias expansion before it has even split simple commands via pipelines and so on? We need to be careful that the interpreter can't get into a situation where aliases expand forever, for example.

[alexflint]

Related to this, aliases can't reference other aliases, so for example:

$ alias a=ls
$ alias b=a
$ b

gives

"a": executable file not found in $PATH

because "b" is expanded to "a" but then "a" is not further expanded