Consider the following problem: I have a function with multiple scalar parameters, and I wish to visualise their influence on the result (a data.frame). My favourite construct for this is plyr::mdply, as illustrated below:

```
weibull <- function(alpha, lambda, time){
data.frame(time = time, value = alpha*lambda*(time^(alpha-1)))
}
library(plyr)
library(ggplot2)
params <- expand.grid(lambda = c(1, 2, 4, 8, 16), alpha = c(0.5, 1))
all <- mdply(params, weibull, time = seq(0, 2, length=100))
ggplot(all, aes(time, value, colour=factor(lambda)))+
facet_wrap(~alpha,scales="free", ncol=2) + geom_line()
```

It's very concise and expressive. Now, have not found the equivalent one-step solution in the tidyverse.

My current strategy is the following,

```
weibull <- function(alpha, lambda, time){
data.frame(time = time, value = alpha*lambda*(time^(alpha-1)))
}
library(ggplot2)
library(tidyverse)
params <- tidyr::crossing(lambda = c(1, 2, 4, 8, 16), alpha = c(0.5, 1))
params %>%
dplyr::mutate(purrr::pmap(., .f = weibull, time = seq(0, 2, length=100))) %>%
tidyr::unnest() %>%
ggplot(aes(time, value, colour=factor(lambda)))+
facet_wrap(~alpha,scales="free", ncol=2) + geom_line()
```

I was hoping initially something like `params %>% purrr::pmap_df(.f = weibull, time = seq(0, 2, length=100))`

would do the job, but it does not keep track of the original parameters.

Another option might be to use the `.id`

variable to merge the original params with the results, but that's a non-trivial extra step.