Stream: helpdesk (published)

Topic: Multiple val types with @eval (maybe?)

Kevin Bonham (May 21 2021 at 16:04):

I'm trying to use a Val type to make some data cleaning operations a bit easier. Here's what I have so far:

struct ObsType{x} end

ObsType(s::Symbol) = ObsType{s}()
ObsType(s::AbstractString) = ObsType(Symbol(s))

_fix_val(val, ::ObsType) = val
_fix_val(val::AbstractFloat, ::ObsType{:timepoint}) = floor(Int, val)
_fix_val(val::AbstractString, ::ObsType{:timepoint}) = parse(Int, val)

This works great, and allows me to do stuff like df[!, col] = _fix_val.(df.col, ObsType{col}), regardless of what col is. Cool.

But now I have several "date-like" column names that I'd like to treat all in the same way. I was thinking I could maybe do this:

# I need this tuple for other purposes, so I thought I could re-use it
const _date_priority = (:collectionDate,
                        :CollectionDate,
                        :assessmentDate,
                        :scanDate,
                        :Date,
                        :momDate)

function _parse_date(datestring)
    occursin("/", datestring) && return Date(datestring, Dates.DateFormat("m/d/y"))
    throw(ArgumentError("Invalid date string: $datestring"))
end

for d in _date_priority
    @show d
    @eval _fix_val(val::AbstractString, ::ObsType{d}) = _parse_date(val)
end

but this gives me UndefVarError: d not defined. I also tried $d, but this gives me UndefVarError: collectionDate not defined. That's the extent of my debugging eval ability :sweat_smile:

Takafumi Arakaki (tkf) (May 21 2021 at 22:05):

I think you need $(QuoteNode(d))

Mason Protter (May 21 2021 at 22:31):

This is basically a quirk of Expr and Symbol inside eval or when returned from macros. You need QuoteNode to signal to julia that the Symbol is meant to be interpreted as a symbol rather than as a variable name.

Takafumi Arakaki (tkf) (May 21 2021 at 22:46):

I tend to think it's more like a universal pain that comes with AST manipulation (and not just Julia's Expr). I imagine you'd need some kind of quote/unquote machinery when dealing with AST in general.

Brian Chen (May 21 2021 at 22:56):

Quasiquoting in particular never fails to give me a headache :P

Mason Protter (May 21 2021 at 23:00):

Yes of course. I just mean that it's a quirk of using elements of the AST in code. Not that we did something wrong with Expr and Symbol

Takafumi Arakaki (tkf) (May 21 2021 at 23:09):

Ah, yes, you mentioned "quirk ... inside eval." Totally agree Expr and Symbol are more quirky in eval.

Last updated: Oct 02 2023 at 04:34 UTC