Unexpected 'else' error when attempting to fit with uninstalled random forest engine

[RichardPilbery]

The problem

When using parsnip v0.1.0 and attempting to fit a random forest model using ranger (which is not installed) an 'unexpected else' error appears.

Reproducible example

remove.packages('ranger')

library(modeldata)
data(cells, package = "modeldata")

library(parsnip)

rf_mod <- 
  rand_forest(trees = 1000) %>% 
  set_engine("ranger") %>%
  set_mode("classification")

set.seed(234)

rf_fit <- rf_mod %>% 
  fit(class ~ angle_ch_1, data = cells[cells$case == 'Train',])

# Error in parse(text = text, keep.source = FALSE) : 
#  <text>:5:5: unexpected 'else'
# 4:     }
# 5:     else
#       ^

Traceback output

20: parse(text = text, keep.source = FALSE)
19: eval(parse(text = text, keep.source = FALSE), envir)
18: .transformer(expr, env)
17: (function (expr) 
    {
        eval_func <- .transformer(expr, env)
        tryCatch(as.character(eval_func), error = function(e) {
            if (is.function(eval_func)) {
                message <- paste0("glue cannot interpolate functions into strings.\n", 
                    "* object '", expr, "' is a function.")
                stop(message, call. = FALSE)
            }
            stop(e)
        })
    })("\n    if (results$treetype == \"Probability estimation\") {\n        res <- colnames(results$predictions)[apply(results$predictions, 1, which.max)]\n    }\n    else {\n        res <- results$predictions\n    }\n    res\n")
16: glue_data(.x = NULL, ..., .sep = .sep, .envir = .envir, .open = .open, 
        .close = .close, .na = .na, .transformer = .transformer, 
        .trim = .trim)
15: glue::glue("This engine requires some package installs: ", glue::glue_collapse(glue::glue("'{x}'"), 
        sep = ", "))
14: is_null(msg)
13: validate_signal_message(message, class)
12: rlang::abort(glue::glue("This engine requires some package installs: ", 
        glue::glue_collapse(glue::glue("'{x}'"), sep = ", ")))
11: check_installs(object)
10: fit.model_spec(., class ~ angle_ch_1, data = cells[cells$case == 
        "Train", ])
9: fit(., class ~ angle_ch_1, data = cells[cells$case == "Train", 
       ])
8: function_list[[k]](value)
7: withVisible(function_list[[k]](value))
6: freduce(value, `_function_list`)
5: `_fseq`(`_lhs`)
4: eval(quote(`_fseq`(`_lhs`)), env, env)
3: eval(quote(`_fseq`(`_lhs`)), env, env)
2: withVisible(eval(quote(`_fseq`(`_lhs`)), env, env))
1: rf_mod %>% fit(class ~ angle_ch_1, data = cells[cells$case == 
       "Train", ])

[github-actions]

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