Add map() to the DSL

[wlandau]

Summary

library(drake)
plan <- drake_plan(
  data = target(
    simulate(data, nrows),
    transform = map(data = c(mtcars, "iris"), nrows = c(48, 64))
  ),
  analysis = target(analyze(data), transform = reduce())
)
plan
#> # A tibble: 3 x 2
#>   target        command                                                    
#>   <chr>         <chr>                                                      
#> 1 data_mtcars_… simulate(mtcars, 48)                                       
#> 2 data_.iris._… "simulate(\"iris\", 64)"                                   
#> 3 analysis      analyze(list(data_mtcars_48 = data_mtcars_48, data_.iris._…

drake_plan_source(plan)
#> drake_plan(
#>   data_mtcars_48 = simulate(mtcars, 48),
#>   data_.iris._64 = simulate("iris", 64),
#>   analysis = analyze(list(data_mtcars_48 = data_mtcars_48, data_.iris._64 = data_.iris._64))
#> )

^{Created on 2019-01-19 by the reprex package (v0.2.1)}

cc @AlexAxthelm re #235 (comment). Fixes #233.

This is a better solution than map_plan() because the DSL lets you work entirely in drake_plan().

Related GitHub issues and pull requests

• Ref: DSL based on dplyr-like verbs? #233

Checklist

• I have read drake's code of conduct, and I agree to follow its rules.
• I have listed any substantial changes in the development news.
• I have added testthat unit tests to tests/testthat to confirm that any new features or functionality work correctly.
• I have tested this pull request locally with devtools::check()
• This pull request is ready for review.
• I think this pull request is ready to merge.

[codecov-io]

Codecov Report

Merging #687 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #687   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          72     72           
  Lines        5694   5705   +11     
=====================================
+ Hits         5694   5705   +11

Impacted Files	Coverage Δ
R/api-plan.R	100% <ø> (ø)	⬆️
R/api-dsl.R	100% <100%> (ø)	⬆️

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