add function for extracting the parameter block of stan code

.Rbuildignore

@@ -12,3 +12,4 @@
 ^vignettes/articles$
 ^doc$
 ^Meta$
+^dev_utils$

DESCRIPTION

@@ -24,7 +24,8 @@ Suggests:
     ggplot2,
     mixtur,
     ggthemes,
-    cowplot
+    cowplot,
+    stringr
 Config/testthat/edition: 3
 Imports: 
     brms,

NAMESPACE

@@ -32,6 +32,7 @@ export(dsdm)
 export(fit_model)
 export(get_model_prior)
 export(get_stancode)
+export(get_stancode_parblock)
 export(get_standata)
 export(k2sd)
 export(mixture2p)

R/helpers-model.R

@@ -540,3 +540,53 @@ get_stancode <- function(formula, data, model, prior=NULL, ...) {
 
   return(stancode)
 }
+
+
+
+#' @title Get the parameter block from a generated Stan code for bmm models
+#' @description A wrapper around `get_stancode()` for models specified with
+#'   `bmm`. Given the `model`, the `data` and the `formula` for the model, this
+#'   function will return just the parameters block. Useful for figuring out
+#'   which paramters you can set initial values on
+#' @param formula An object of class `brmsformula`. A symbolic description of
+#'   the model to be fitted.
+#' @param data An object of class data.frame, containing data of all variables
+#'   used in the model. The names of the variables must match the variable names
+#'   passed to the `bmmmodel` object for required argurments.
+#' @param model A description of the model to be fitted. This is a call to a
+#'   `bmmmodel` such as `mixture3p()` function. Every model function has a
+#'   number of required arguments which need to be specified within the function
+#'   call. Call [supported_models()] to see the list of supported models and
+#'   their required arguments
+#' @param prior One or more `brmsprior` objects created by [brms::set_prior()]
+#'   or related functions and combined using the c method or the + operator. See
+#'   also [get_model_prior()] for more help. Not necessary for the default model
+#'   fitting, but you can provide prior constraints to model parameters
+#' @param ... Further arguments passed to [brms::make_stancode()]. See the
+#'   description of [brms::make_stancode()] for more details
+#'
+#' @returns A character string containing the parameter block of fully commented
+#'   Stan code to fit a bmm model.
+#'
+#'
+#' @seealso [supported_models()], [get_stancode()]
+#'
+#' @export
+get_stancode_parblock <- function(formula, data, model, prior=NULL, ...) {
+  stancode <- get_stancode(formula, data, model, prior, ...)
+  parblock <- .extract_parblock(stancode)
+  return(parblock)
+}
+
+
+#' @title Extract the parameter block from the Stan code
+#' @description Given the Stan code for a model, this function will extract the
+#'   parameter block from the Stan code
+#' @param stancode A character string containing the fully commented Stan code
+#' @noRd
+.extract_parblock <- function(stancode) {
+  parblock <- stringr::str_match(as.character(stancode),
+                                 "(?s)parameters \\{\\n(.*?)\\}\\ntransformed")[,2]
+  class(parblock) <- class(stancode)
+  return(parblock)
+}