I() operator constraint can now also take character strings and vectors.

Author: krivit

DESCRIPTION

@@ -1,6 +1,6 @@
 Package: ergm
-Version: 4.12.1-7996
-Date: 2026-02-24
+Version: 4.12.1-7997
+Date: 2026-02-25
 Title: Fit, Simulate and Diagnose Exponential-Family Models for Networks
 Authors@R: c(
              person(c("Mark", "S."), "Handcock", role=c("aut"), email="handcock@stat.ucla.edu"),

R/InitErgmConstraint.operator.R

@@ -73,20 +73,34 @@ InitErgmConstraint.Dyads<-function(nw, arglist, ..., verify_dind = TRUE){
 #' # I(formula)
 #' @param formula a constraints formula
 #'
-#' @note `formula` can also be a [term_list].
+#' @note `formula` can also be a [`term_list`] or [`character`]. In
+#'   the latter case, if there are multiple strings, they will be
+#'   concatenated with `+`, and if they do not start with "~", one
+#'   will be prepended. Its environment will be inherited from the
+#'   top-level formula.
 #'
 #' @seealso [base::I()] (a.k.a. `AsIs`)
 #'
 #' @template ergmConstraint-general
 #'
 #' @concept operator
-InitErgmConstraint.I <- function(nw, arglist, ...) {
+InitErgmConstraint.I <- function(nw, arglist, ..., env) {
   a <- check.ErgmTerm(nw, arglist,
                       varnames = c("formula"),
-                      vartypes = c("formula"),
+                      vartypes = c("formula,character,term_list"),
                       defaultvalues = list(NULL),
                       required = c(TRUE))
 
+  f <- a$formula
+
+  if (is.character(f)) {
+    if (length(f) > 1) f <- paste(f, collapse = " + ")
+
+    if (!startsWith(trimws(f, "left"), "~")) f <- paste0("~", f)
+
+    f <- as.formula(f, env)
+  }
+
   ergm_conlist(a$formula, nw, ...)
 }