0.4.8 lets `a.[1]` parse and lower

[LilithHafner]

This came up at JuliaLang/julia#53119

@LilithHafnerBot bisect()

using JuliaSyntax
JuliaSyntax.parse("a.[1]")

[savq]

Note that this was allowed in the flisp parser.

julia +lts -E ':(a.[1])'                                                                                                                                      
:(a.:([1]))

[LilithHafner]

There was a bug in the bisector backend. Should hopefully be fixed now.

@LilithHafnerBot bisect()

using JuliaSyntax
JuliaSyntax.parse(Expr, "a.[1]")

[LilithHafnerBot]

❌ Bisect failed

Commit	stdout
b1c6d54	a.:([1])
a6f2d15	a.:([1])

[LilithHafner]

@LilithHafnerBot bisect()

using JuliaSyntax
JuliaSyntax.enable_in_core!()
eval(Meta.parse("""
Meta.lower(Main, :(a.[1]))
"""))

[LilithHafnerBot]

✅ Bisect succeeded! The first new commit is 296cd5e

Commit	stdout
b1c6d54	$(Expr(:error, "invalid syntax "a.[1]""))
e3e447d	$(Expr(:error, "invalid syntax "a.[1]""))
8731bab	$(Expr(:error, "invalid syntax "a.[1]""))
6da0fc4	$(Expr(:error, "invalid syntax "a.[1]""))
5aad812	$(Expr(:error, "invalid syntax "a.[1]""))
296cd5e	$(Expr(:thunk, CodeInfo(⏎ @ none within top-level scope⏎1 ─ %1 = Base.getproperty(a, $(Quote...
df84e02	$(Expr(:thunk, CodeInfo(⏎ @ none within top-level scope⏎1 ─ %1 = Base.getproperty(a, $(Quote...
a6f2d15	$(Expr(:thunk, CodeInfo(⏎ @ none within top-level scope⏎1 ─ %1 = Base.getproperty(a, $(Quote...

[LilithHafner]

@LilithHafnerBot bisect()

using JuliaSyntax
dump(JuliaSyntax.parse(Expr, "a.[1]"))

[LilithHafnerBot]

✅ Bisect succeeded! The first new commit is 296cd5e

Commit	stdout
b1c6d54	Expr⏎ head: Symbol .⏎ args: Array{Any}((2,))⏎ 1: Symbol a⏎ 2: Expr⏎ head: Symbol quo...
e3e447d	Expr⏎ head: Symbol .⏎ args: Array{Any}((2,))⏎ 1: Symbol a⏎ 2: Expr⏎ head: Symbol quo...
8731bab	Expr⏎ head: Symbol .⏎ args: Array{Any}((2,))⏎ 1: Symbol a⏎ 2: Expr⏎ head: Symbol quo...
6da0fc4	Expr⏎ head: Symbol .⏎ args: Array{Any}((2,))⏎ 1: Symbol a⏎ 2: Expr⏎ head: Symbol quo...
5aad812	Expr⏎ head: Symbol .⏎ args: Array{Any}((2,))⏎ 1: Symbol a⏎ 2: Expr⏎ head: Symbol quo...
296cd5e	Expr⏎ head: Symbol .⏎ args: Array{Any}((2,))⏎ 1: Symbol a⏎ 2: QuoteNode⏎ value: Expr...
df84e02	Expr⏎ head: Symbol .⏎ args: Array{Any}((2,))⏎ 1: Symbol a⏎ 2: QuoteNode⏎ value: Expr...
a6f2d15	Expr⏎ head: Symbol .⏎ args: Array{Any}((2,))⏎ 1: Symbol a⏎ 2: QuoteNode⏎ value: Expr...

[LilithHafner]

Alas, @savq, the old and new parsing results both print to :(a.:([1])), but they are not the same.

julia> old = Expr(:., :a, Expr(:quote, :([1])))
:(a.:([1]))

julia> new = Expr(:., :a, QuoteNode(:([1])))
:(a.:([1]))

julia> old == new
false

julia> Meta.lower(Main, old)
:($(Expr(:error, "invalid syntax \"a.[1]\"")))

julia> Meta.lower(Main, new)
:($(Expr(:thunk, CodeInfo(
    @ none within `top-level scope'
1 ─ %1 = Base.getproperty(a, $(QuoteNode(:([1]))))
└──      return %1
))))

From the commit message, this looks intentional; perhaps the appropriate fix is to prohibit this in lowering (and possibly gate the new parsing to new versions of Julia)

[pfitzseb]

Are Expr(:quote, ...) and QuoteNode(...) semantically identical?

[c42f]

Are Expr(:quote, ...) and QuoteNode(...) semantically identical?

No, but the difference is subtle.

• quote does $ interpolation (in lisp, this is called quasiquote)
• QuoteNode (or equivalently, Expr(:inert)) doesn't participate in $ interpolation (somewhat confusingly, this is called quote in lisp)

[c42f]

Note that the old parser uses QuoteNode in the special case that $ interpolation comes after the .:

julia> dump(JuliaSyntax.fl_parse(raw"x.$y"))
Expr
  head: Symbol .
  args: Array{Any}((2,))
    1: Symbol x
    2: QuoteNode
      value: Expr
        head: Symbol $
        args: Array{Any}((1,))
          1: Symbol y

QuoteNode rather than quote is required in this case so that the following works

julia> let
           field_name = :a
           quote
               x.$field_name
           end
       end
quote
    #= REPL[10]:4 =#
    x.a
end

But in the old parser, the following doesn't work because use of quote for anything other than $ after the x. means that the $field_name is double quoted (ie, "inside two quotes") in the Expr tree:

julia> let
           field_name = :a
           quote
               x.[$field_name]
           end
       end
quote
    #= REPL[11]:4 =#
    x.:([$(Expr(:$, :field_name))])
end

However, after #324, we have

julia> JuliaSyntax.enable_in_core!()

julia> let
           field_name = :a
           quote
               x.[$field_name]
           end
       end
quote
    #= REPL[13]:4 =#
    x.:([a])
end