Handle non-vector sub-targets

[wlandau]

Prework

• Read and abide by drake's code of conduct.

Description

#1105 made dynamic branching a breeze for most use cases, but now drake is less forgiving about non-vector sub-targets.

To quote Hadley in r-lib/vctrs#321:

vctrs is about vectors, not non-vectors

And I totally forgot about it in #1105.

library(drake)
library(gapminder)
library(tidyverse)

# Split the Gapminder data by continent.
gapminder_continents <- function() {
  gapminder %>%
    mutate(gdpPercap = scale(gdpPercap)) %>%
    split(f = .$continent)
}

# Fit a model to a continent.
fit_model <- function(continent_data) {
  lm(formula = gdpPercap ~ year, data = continent_data[[1]])
}

plan <- drake_plan(
  continents = gapminder_continents(),
  model = target(fit_model(continents), dynamic = map(continents))
)

make(plan)
#> target continents
#> dynamic model
#> subtarget model_23022788
#> subtarget model_e020ff00
#> subtarget model_53c1b086
#> subtarget model_0fc4392a
#> subtarget model_8b2d2fd5
#> aggregate model

readd(model)
#> Error: `..1` must be a vector, not a `lm` object

^{Created on 2020-01-18 by the reprex package (v0.3.0)}

The error comes from trying to aggregate non-vectors.

x <- lm(mpg ~ cyl, data = mtcars)
y <- lm(mpg ~ wt, data = mtcars)
vctrs::vec_c(x, y)
#> Error : `..1` must be a vector, not a `lm` object

^{Created on 2020-01-18 by the reprex package (v0.3.0)}

We need some special error handling to catch non-vectors.

safe_vec_c <- function(...) {
  tryCatch(
    vctrs::vec_c(x, y),
    vctrs_error_scalar_type = function(e) {
      list(x, y)
    },
    error = function(e) {
      stop(e)
    }
  )
}

x <- lm(mpg ~ cyl, data = mtcars)
y <- lm(mpg ~ wt, data = mtcars)

safe_vec_c(x, y)
#> [[1]]
#> 
#> Call:
#> lm(formula = mpg ~ cyl, data = mtcars)
#> 
#> Coefficients:
#> (Intercept)          cyl  
#>      37.885       -2.876  
#> 
#> 
#> [[2]]
#> 
#> Call:
#> lm(formula = mpg ~ wt, data = mtcars)
#> 
#> Coefficients:
#> (Intercept)           wt  
#>      37.285       -5.344

^{Created on 2020-01-18 by the reprex package (v0.3.0)}