diff --git a/NAMESPACE b/NAMESPACE
index acec56761..934de1e63 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -159,6 +159,7 @@ S3method(rxSolve,rxode2tos)
 S3method(rxTrans,character)
 S3method(rxTrans,default)
 S3method(rxUdfUi,default)
+S3method(rxUdfUi,expit)
 S3method(rxUdfUi,linMod)
 S3method(rxUdfUi,linMod0)
 S3method(rxUdfUi,linModA)
@@ -169,6 +170,7 @@ S3method(rxUdfUi,linModD)
 S3method(rxUdfUi,linModD0)
 S3method(rxUdfUi,linModM)
 S3method(rxUdfUi,linModM0)
+S3method(rxUdfUi,logit)
 S3method(rxUdfUi,ribeta)
 S3method(rxUdfUi,ribinom)
 S3method(rxUdfUi,ricauchy)
@@ -341,6 +343,7 @@ export(.udfEnvSet)
 export(.udfEnvSetUdf)
 export(.udfExists)
 export(.udfMd5Info)
+export(.uiArg)
 export(.useUtf)
 export(.vecDf)
 export(ELU)
diff --git a/NEWS.md b/NEWS.md
index dc9722cc0..fd41347c0 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,14 @@
 # rxode2 (development version)
 
+- Add `logit`/`expit` named expressions, that is `logit(x, high=20)`
+  becomes `logit(x, 0, 20)` in ui models.
+
+- Updated random ui models like `rxnorm(sd=10)` to accept complex
+  numeric expressions like `rxnorm(sd=10+1)`.
+
+- Updated random ui models to accept complex non-numeric expressions
+  like `rxnorm(sd=a+b)`
+
 - Rework the `tad()` and related functions so they use the same
   interface as compartments (this way they do not depend on the order
   of compartments); See #815.  For mu-referencing, Also allow dummy
diff --git a/R/build.R b/R/build.R
index 8ca93c905..1e0f8a5af 100644
--- a/R/build.R
+++ b/R/build.R
@@ -68,12 +68,9 @@ d/dt(blood)     = a*intestine - b*blood
               .arg <- str2lang(arg)
               .ret <- bquote({
                 .(.dotArg) <- as.character(substitute(.(.arg)))
+                .dp <- deparse1(substitute(.(.arg)))
                 .tmp <- suppressWarnings(try(force(.(.arg)), silent=TRUE))
-                if (!inherits(.tmp, "try-error")) {
-                  if (is.character(.tmp)) {
-                    .(.dotArg) <- .(.arg)
-                  }
-                }
+                .(.dotArg) <- .uiArg(.(.dotArg), .tmp, .dp)
               })
               lapply(seq_along(.ret)[-1], function(i) {
                 .ret[[i]]
@@ -111,7 +108,9 @@ d/dt(blood)     = a*intestine - b*blood
                "rxbinom"=c("size", "prob"))
   .lst2 <- .lst
   names(.lst2) <- gsub("rx", "ri", names(.lst2))
-  .lst <- c(.lst, .lst2)
+  .lst <- c(.lst, .lst2,
+            list("logit"=c("x"=NA, "low"=0, "high"=1),
+                 "expit"=c("x"=NA, "low"=0, "high"=1)))
   paste0("# This file is generated by .generateRandomUiFuns() in build.R\n## nocov start\n",
          vapply(names(.lst), function(fun) {
            if (is.null(names(.lst[[fun]]))) {
diff --git a/R/rudfui.R b/R/rudfui.R
index c98580313..13ddc07ee 100644
--- a/R/rudfui.R
+++ b/R/rudfui.R
@@ -248,7 +248,8 @@ rxUdfUiParsing <- function() {
         }
         expr <- .t
       } else {
-        stop("rxode2 ui user function '", .c, "' failed to produce code that could be parsed in the",
+
+        stop("rxode2 ui user function '", .c, "' failed to produce code that could be parsed",
              call.=FALSE)
       }
       .handleUdifUiBeforeOrAfter("before", .e, env, .c)
diff --git a/R/rxrandomui.R b/R/rxrandomui.R
index 5c1b5da80..b01315e59 100644
--- a/R/rxrandomui.R
+++ b/R/rxrandomui.R
@@ -2,19 +2,13 @@
 ## nocov start
 .rxnorm <- function(mean = 0, sd = 1) {
     .mean <- as.character(substitute(mean))
+    .dp <- deparse1(substitute(mean))
     .tmp <- suppressWarnings(try(force(mean), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .mean <- mean
-        }
-    }
+    .mean <- .uiArg(.mean, .tmp, .dp)
     .sd <- as.character(substitute(sd))
+    .dp <- deparse1(substitute(sd))
     .tmp <- suppressWarnings(try(force(sd), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .sd <- sd
-        }
-    }
+    .sd <- .uiArg(.sd, .tmp, .dp)
     list(replace = paste0("rxnorm(", .mean, ", ", .sd, ")"))
 }
 
@@ -26,12 +20,9 @@ rxUdfUi.rxnorm <- rxUdfUi.rxpois
 ## nocov start
 .rxpois <- function(lambda) {
     .lambda <- as.character(substitute(lambda))
+    .dp <- deparse1(substitute(lambda))
     .tmp <- suppressWarnings(try(force(lambda), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .lambda <- lambda
-        }
-    }
+    .lambda <- .uiArg(.lambda, .tmp, .dp)
     list(replace = paste0("rxpois(", .lambda, ")"))
 }
 
@@ -43,12 +34,9 @@ rxUdfUi.rxpois <- rxUdfUi.rxpois
 ## nocov start
 .rxt <- function(df) {
     .df <- as.character(substitute(df))
+    .dp <- deparse1(substitute(df))
     .tmp <- suppressWarnings(try(force(df), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .df <- df
-        }
-    }
+    .df <- .uiArg(.df, .tmp, .dp)
     list(replace = paste0("rxt(", .df, ")"))
 }
 
@@ -60,19 +48,13 @@ rxUdfUi.rxt <- rxUdfUi.rxpois
 ## nocov start
 .rxunif <- function(min = 0, max = 1) {
     .min <- as.character(substitute(min))
+    .dp <- deparse1(substitute(min))
     .tmp <- suppressWarnings(try(force(min), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .min <- min
-        }
-    }
+    .min <- .uiArg(.min, .tmp, .dp)
     .max <- as.character(substitute(max))
+    .dp <- deparse1(substitute(max))
     .tmp <- suppressWarnings(try(force(max), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .max <- max
-        }
-    }
+    .max <- .uiArg(.max, .tmp, .dp)
     list(replace = paste0("rxunif(", .min, ", ", .max, ")"))
 }
 
@@ -84,19 +66,13 @@ rxUdfUi.rxunif <- rxUdfUi.rxpois
 ## nocov start
 .rxweibull <- function(shape, scale = 1) {
     .shape <- as.character(substitute(shape))
+    .dp <- deparse1(substitute(shape))
     .tmp <- suppressWarnings(try(force(shape), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .shape <- shape
-        }
-    }
+    .shape <- .uiArg(.shape, .tmp, .dp)
     .scale <- as.character(substitute(scale))
+    .dp <- deparse1(substitute(scale))
     .tmp <- suppressWarnings(try(force(scale), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .scale <- scale
-        }
-    }
+    .scale <- .uiArg(.scale, .tmp, .dp)
     list(replace = paste0("rxweibull(", .shape, ", ", .scale, 
         ")"))
 }
@@ -109,12 +85,9 @@ rxUdfUi.rxweibull <- rxUdfUi.rxpois
 ## nocov start
 .rxgeom <- function(prob) {
     .prob <- as.character(substitute(prob))
+    .dp <- deparse1(substitute(prob))
     .tmp <- suppressWarnings(try(force(prob), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .prob <- prob
-        }
-    }
+    .prob <- .uiArg(.prob, .tmp, .dp)
     list(replace = paste0("rxgeom(", .prob, ")"))
 }
 
@@ -126,19 +99,13 @@ rxUdfUi.rxgeom <- rxUdfUi.rxpois
 ## nocov start
 .rxbeta <- function(shape1, shape2) {
     .shape1 <- as.character(substitute(shape1))
+    .dp <- deparse1(substitute(shape1))
     .tmp <- suppressWarnings(try(force(shape1), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .shape1 <- shape1
-        }
-    }
+    .shape1 <- .uiArg(.shape1, .tmp, .dp)
     .shape2 <- as.character(substitute(shape2))
+    .dp <- deparse1(substitute(shape2))
     .tmp <- suppressWarnings(try(force(shape2), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .shape2 <- shape2
-        }
-    }
+    .shape2 <- .uiArg(.shape2, .tmp, .dp)
     list(replace = paste0("rxbeta(", .shape1, ", ", .shape2, 
         ")"))
 }
@@ -151,19 +118,13 @@ rxUdfUi.rxbeta <- rxUdfUi.rxpois
 ## nocov start
 .rxgamma <- function(shape, rate = 1) {
     .shape <- as.character(substitute(shape))
+    .dp <- deparse1(substitute(shape))
     .tmp <- suppressWarnings(try(force(shape), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .shape <- shape
-        }
-    }
+    .shape <- .uiArg(.shape, .tmp, .dp)
     .rate <- as.character(substitute(rate))
+    .dp <- deparse1(substitute(rate))
     .tmp <- suppressWarnings(try(force(rate), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .rate <- rate
-        }
-    }
+    .rate <- .uiArg(.rate, .tmp, .dp)
     list(replace = paste0("rxgamma(", .shape, ", ", .rate, ")"))
 }
 
@@ -175,19 +136,13 @@ rxUdfUi.rxgamma <- rxUdfUi.rxpois
 ## nocov start
 .rxf <- function(df1, df2) {
     .df1 <- as.character(substitute(df1))
+    .dp <- deparse1(substitute(df1))
     .tmp <- suppressWarnings(try(force(df1), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .df1 <- df1
-        }
-    }
+    .df1 <- .uiArg(.df1, .tmp, .dp)
     .df2 <- as.character(substitute(df2))
+    .dp <- deparse1(substitute(df2))
     .tmp <- suppressWarnings(try(force(df2), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .df2 <- df2
-        }
-    }
+    .df2 <- .uiArg(.df2, .tmp, .dp)
     list(replace = paste0("rxf(", .df1, ", ", .df2, ")"))
 }
 
@@ -199,12 +154,9 @@ rxUdfUi.rxf <- rxUdfUi.rxpois
 ## nocov start
 .rxexp <- function(rate) {
     .rate <- as.character(substitute(rate))
+    .dp <- deparse1(substitute(rate))
     .tmp <- suppressWarnings(try(force(rate), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .rate <- rate
-        }
-    }
+    .rate <- .uiArg(.rate, .tmp, .dp)
     list(replace = paste0("rxexp(", .rate, ")"))
 }
 
@@ -216,12 +168,9 @@ rxUdfUi.rxexp <- rxUdfUi.rxpois
 ## nocov start
 .rxchisq <- function(df) {
     .df <- as.character(substitute(df))
+    .dp <- deparse1(substitute(df))
     .tmp <- suppressWarnings(try(force(df), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .df <- df
-        }
-    }
+    .df <- .uiArg(.df, .tmp, .dp)
     list(replace = paste0("rxchisq(", .df, ")"))
 }
 
@@ -233,19 +182,13 @@ rxUdfUi.rxchisq <- rxUdfUi.rxpois
 ## nocov start
 .rxcauchy <- function(location = 0, scale = 1) {
     .location <- as.character(substitute(location))
+    .dp <- deparse1(substitute(location))
     .tmp <- suppressWarnings(try(force(location), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .location <- location
-        }
-    }
+    .location <- .uiArg(.location, .tmp, .dp)
     .scale <- as.character(substitute(scale))
+    .dp <- deparse1(substitute(scale))
     .tmp <- suppressWarnings(try(force(scale), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .scale <- scale
-        }
-    }
+    .scale <- .uiArg(.scale, .tmp, .dp)
     list(replace = paste0("rxcauchy(", .location, ", ", .scale, 
         ")"))
 }
@@ -258,19 +201,13 @@ rxUdfUi.rxcauchy <- rxUdfUi.rxpois
 ## nocov start
 .rxbinom <- function(size, prob) {
     .size <- as.character(substitute(size))
+    .dp <- deparse1(substitute(size))
     .tmp <- suppressWarnings(try(force(size), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .size <- size
-        }
-    }
+    .size <- .uiArg(.size, .tmp, .dp)
     .prob <- as.character(substitute(prob))
+    .dp <- deparse1(substitute(prob))
     .tmp <- suppressWarnings(try(force(prob), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .prob <- prob
-        }
-    }
+    .prob <- .uiArg(.prob, .tmp, .dp)
     list(replace = paste0("rxbinom(", .size, ", ", .prob, ")"))
 }
 
@@ -282,19 +219,13 @@ rxUdfUi.rxbinom <- rxUdfUi.rxpois
 ## nocov start
 .rinorm <- function(mean = 0, sd = 1) {
     .mean <- as.character(substitute(mean))
+    .dp <- deparse1(substitute(mean))
     .tmp <- suppressWarnings(try(force(mean), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .mean <- mean
-        }
-    }
+    .mean <- .uiArg(.mean, .tmp, .dp)
     .sd <- as.character(substitute(sd))
+    .dp <- deparse1(substitute(sd))
     .tmp <- suppressWarnings(try(force(sd), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .sd <- sd
-        }
-    }
+    .sd <- .uiArg(.sd, .tmp, .dp)
     list(replace = paste0("rinorm(", .mean, ", ", .sd, ")"))
 }
 
@@ -306,12 +237,9 @@ rxUdfUi.rinorm <- rxUdfUi.rxpois
 ## nocov start
 .ripois <- function(lambda) {
     .lambda <- as.character(substitute(lambda))
+    .dp <- deparse1(substitute(lambda))
     .tmp <- suppressWarnings(try(force(lambda), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .lambda <- lambda
-        }
-    }
+    .lambda <- .uiArg(.lambda, .tmp, .dp)
     list(replace = paste0("ripois(", .lambda, ")"))
 }
 
@@ -323,12 +251,9 @@ rxUdfUi.ripois <- rxUdfUi.rxpois
 ## nocov start
 .rit <- function(df) {
     .df <- as.character(substitute(df))
+    .dp <- deparse1(substitute(df))
     .tmp <- suppressWarnings(try(force(df), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .df <- df
-        }
-    }
+    .df <- .uiArg(.df, .tmp, .dp)
     list(replace = paste0("rit(", .df, ")"))
 }
 
@@ -340,19 +265,13 @@ rxUdfUi.rit <- rxUdfUi.rxpois
 ## nocov start
 .riunif <- function(min = 0, max = 1) {
     .min <- as.character(substitute(min))
+    .dp <- deparse1(substitute(min))
     .tmp <- suppressWarnings(try(force(min), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .min <- min
-        }
-    }
+    .min <- .uiArg(.min, .tmp, .dp)
     .max <- as.character(substitute(max))
+    .dp <- deparse1(substitute(max))
     .tmp <- suppressWarnings(try(force(max), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .max <- max
-        }
-    }
+    .max <- .uiArg(.max, .tmp, .dp)
     list(replace = paste0("riunif(", .min, ", ", .max, ")"))
 }
 
@@ -364,19 +283,13 @@ rxUdfUi.riunif <- rxUdfUi.rxpois
 ## nocov start
 .riweibull <- function(shape, scale = 1) {
     .shape <- as.character(substitute(shape))
+    .dp <- deparse1(substitute(shape))
     .tmp <- suppressWarnings(try(force(shape), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .shape <- shape
-        }
-    }
+    .shape <- .uiArg(.shape, .tmp, .dp)
     .scale <- as.character(substitute(scale))
+    .dp <- deparse1(substitute(scale))
     .tmp <- suppressWarnings(try(force(scale), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .scale <- scale
-        }
-    }
+    .scale <- .uiArg(.scale, .tmp, .dp)
     list(replace = paste0("riweibull(", .shape, ", ", .scale, 
         ")"))
 }
@@ -389,12 +302,9 @@ rxUdfUi.riweibull <- rxUdfUi.rxpois
 ## nocov start
 .rigeom <- function(prob) {
     .prob <- as.character(substitute(prob))
+    .dp <- deparse1(substitute(prob))
     .tmp <- suppressWarnings(try(force(prob), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .prob <- prob
-        }
-    }
+    .prob <- .uiArg(.prob, .tmp, .dp)
     list(replace = paste0("rigeom(", .prob, ")"))
 }
 
@@ -406,19 +316,13 @@ rxUdfUi.rigeom <- rxUdfUi.rxpois
 ## nocov start
 .ribeta <- function(shape1, shape2) {
     .shape1 <- as.character(substitute(shape1))
+    .dp <- deparse1(substitute(shape1))
     .tmp <- suppressWarnings(try(force(shape1), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .shape1 <- shape1
-        }
-    }
+    .shape1 <- .uiArg(.shape1, .tmp, .dp)
     .shape2 <- as.character(substitute(shape2))
+    .dp <- deparse1(substitute(shape2))
     .tmp <- suppressWarnings(try(force(shape2), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .shape2 <- shape2
-        }
-    }
+    .shape2 <- .uiArg(.shape2, .tmp, .dp)
     list(replace = paste0("ribeta(", .shape1, ", ", .shape2, 
         ")"))
 }
@@ -431,19 +335,13 @@ rxUdfUi.ribeta <- rxUdfUi.rxpois
 ## nocov start
 .rigamma <- function(shape, rate = 1) {
     .shape <- as.character(substitute(shape))
+    .dp <- deparse1(substitute(shape))
     .tmp <- suppressWarnings(try(force(shape), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .shape <- shape
-        }
-    }
+    .shape <- .uiArg(.shape, .tmp, .dp)
     .rate <- as.character(substitute(rate))
+    .dp <- deparse1(substitute(rate))
     .tmp <- suppressWarnings(try(force(rate), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .rate <- rate
-        }
-    }
+    .rate <- .uiArg(.rate, .tmp, .dp)
     list(replace = paste0("rigamma(", .shape, ", ", .rate, ")"))
 }
 
@@ -455,19 +353,13 @@ rxUdfUi.rigamma <- rxUdfUi.rxpois
 ## nocov start
 .rif <- function(df1, df2) {
     .df1 <- as.character(substitute(df1))
+    .dp <- deparse1(substitute(df1))
     .tmp <- suppressWarnings(try(force(df1), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .df1 <- df1
-        }
-    }
+    .df1 <- .uiArg(.df1, .tmp, .dp)
     .df2 <- as.character(substitute(df2))
+    .dp <- deparse1(substitute(df2))
     .tmp <- suppressWarnings(try(force(df2), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .df2 <- df2
-        }
-    }
+    .df2 <- .uiArg(.df2, .tmp, .dp)
     list(replace = paste0("rif(", .df1, ", ", .df2, ")"))
 }
 
@@ -479,12 +371,9 @@ rxUdfUi.rif <- rxUdfUi.rxpois
 ## nocov start
 .riexp <- function(rate) {
     .rate <- as.character(substitute(rate))
+    .dp <- deparse1(substitute(rate))
     .tmp <- suppressWarnings(try(force(rate), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .rate <- rate
-        }
-    }
+    .rate <- .uiArg(.rate, .tmp, .dp)
     list(replace = paste0("riexp(", .rate, ")"))
 }
 
@@ -496,12 +385,9 @@ rxUdfUi.riexp <- rxUdfUi.rxpois
 ## nocov start
 .richisq <- function(df) {
     .df <- as.character(substitute(df))
+    .dp <- deparse1(substitute(df))
     .tmp <- suppressWarnings(try(force(df), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .df <- df
-        }
-    }
+    .df <- .uiArg(.df, .tmp, .dp)
     list(replace = paste0("richisq(", .df, ")"))
 }
 
@@ -513,19 +399,13 @@ rxUdfUi.richisq <- rxUdfUi.rxpois
 ## nocov start
 .ricauchy <- function(location = 0, scale = 1) {
     .location <- as.character(substitute(location))
+    .dp <- deparse1(substitute(location))
     .tmp <- suppressWarnings(try(force(location), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .location <- location
-        }
-    }
+    .location <- .uiArg(.location, .tmp, .dp)
     .scale <- as.character(substitute(scale))
+    .dp <- deparse1(substitute(scale))
     .tmp <- suppressWarnings(try(force(scale), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .scale <- scale
-        }
-    }
+    .scale <- .uiArg(.scale, .tmp, .dp)
     list(replace = paste0("ricauchy(", .location, ", ", .scale, 
         ")"))
 }
@@ -538,19 +418,13 @@ rxUdfUi.ricauchy <- rxUdfUi.rxpois
 ## nocov start
 .ribinom <- function(size, prob) {
     .size <- as.character(substitute(size))
+    .dp <- deparse1(substitute(size))
     .tmp <- suppressWarnings(try(force(size), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .size <- size
-        }
-    }
+    .size <- .uiArg(.size, .tmp, .dp)
     .prob <- as.character(substitute(prob))
+    .dp <- deparse1(substitute(prob))
     .tmp <- suppressWarnings(try(force(prob), silent = TRUE))
-    if (!inherits(.tmp, "try-error")) {
-        if (is.character(.tmp)) {
-            .prob <- prob
-        }
-    }
+    .prob <- .uiArg(.prob, .tmp, .dp)
     list(replace = paste0("ribinom(", .size, ", ", .prob, ")"))
 }
 
@@ -558,3 +432,49 @@ rxUdfUi.ricauchy <- rxUdfUi.rxpois
 rxUdfUi.ribinom <- rxUdfUi.rxpois
 
 ## nocov end
+# This file is generated by .generateRandomUiFuns() in build.R
+## nocov start
+.logit <- function(x, low = 0, high = 1) {
+    .x <- as.character(substitute(x))
+    .dp <- deparse1(substitute(x))
+    .tmp <- suppressWarnings(try(force(x), silent = TRUE))
+    .x <- .uiArg(.x, .tmp, .dp)
+    .low <- as.character(substitute(low))
+    .dp <- deparse1(substitute(low))
+    .tmp <- suppressWarnings(try(force(low), silent = TRUE))
+    .low <- .uiArg(.low, .tmp, .dp)
+    .high <- as.character(substitute(high))
+    .dp <- deparse1(substitute(high))
+    .tmp <- suppressWarnings(try(force(high), silent = TRUE))
+    .high <- .uiArg(.high, .tmp, .dp)
+    list(replace = paste0("logit(", .x, ", ", .low, ", ", .high, 
+        ")"))
+}
+
+#'@export
+rxUdfUi.logit <- rxUdfUi.rxpois
+
+## nocov end
+# This file is generated by .generateRandomUiFuns() in build.R
+## nocov start
+.expit <- function(x, low = 0, high = 1) {
+    .x <- as.character(substitute(x))
+    .dp <- deparse1(substitute(x))
+    .tmp <- suppressWarnings(try(force(x), silent = TRUE))
+    .x <- .uiArg(.x, .tmp, .dp)
+    .low <- as.character(substitute(low))
+    .dp <- deparse1(substitute(low))
+    .tmp <- suppressWarnings(try(force(low), silent = TRUE))
+    .low <- .uiArg(.low, .tmp, .dp)
+    .high <- as.character(substitute(high))
+    .dp <- deparse1(substitute(high))
+    .tmp <- suppressWarnings(try(force(high), silent = TRUE))
+    .high <- .uiArg(.high, .tmp, .dp)
+    list(replace = paste0("expit(", .x, ", ", .low, ", ", .high, 
+        ")"))
+}
+
+#'@export
+rxUdfUi.expit <- rxUdfUi.rxpois
+
+## nocov end
diff --git a/R/utils.R b/R/utils.R
index 4e8090347..05684e946 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -507,6 +507,37 @@ expit <- function(alpha, low = 0, high = 1) {
   .rxTransform(alpha, 1.0, low, high, 4L, TRUE)
 }
 
+#' Handle arguments for ui functions
+#'
+#' Note this is an internal function but it is exported in case it is
+#' useful.
+#'
+#' @param char This is the character equivalent of the argument
+#' @param f This is the forced equivalent of the argument
+#' @param dp This is deparsed expression
+#' @return character representing the underlying rxode2 code for the argument
+#' @export
+#' @author Matthew L. Fidler
+#' @keywords internal
+#' @examples
+#'
+#' .uiArg("1.0", 1.0, "1.0")
+.uiArg <- function(char, f, dp) {
+  if (!inherits(f, "try-error")) {
+    if (is.numeric(f)) {
+      return(as.character(f))
+    }
+    if (is.character(f)) {
+      return(f)
+    }
+  }
+  if (length(char) > 1) {
+    dp
+  } else {
+    char
+  }
+}
+
 #' @rdname logit
 #' @export
 logitNormInfo <- function(mean = 0, sd = 1, low = 0, high = 1, abs.tol = 1e-6, ...) {
diff --git a/man/dot-uiArg.Rd b/man/dot-uiArg.Rd
new file mode 100644
index 000000000..c12288d5c
--- /dev/null
+++ b/man/dot-uiArg.Rd
@@ -0,0 +1,30 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/utils.R
+\name{.uiArg}
+\alias{.uiArg}
+\title{Handle arguments for ui functions}
+\usage{
+.uiArg(char, f, dp)
+}
+\arguments{
+\item{char}{This is the character equivalent of the argument}
+
+\item{f}{This is the forced equivalent of the argument}
+
+\item{dp}{This is deparsed expression}
+}
+\value{
+character representing the underlying rxode2 code for the argument
+}
+\description{
+Note this is an internal function but it is exported in case it is
+useful.
+}
+\examples{
+
+.uiArg("1.0", 1.0, "1.0")
+}
+\author{
+Matthew L. Fidler
+}
+\keyword{internal}
diff --git a/tests/testthat/test-logit.R b/tests/testthat/test-logit.R
index 0b3573985..9e6277338 100644
--- a/tests/testthat/test-logit.R
+++ b/tests/testthat/test-logit.R
@@ -1,4 +1,37 @@
 rxTest({
+
+  test_that("logit ui test", {
+
+    f <- function() {
+      model({
+        a <- logit(x, high=10)
+      })
+    }
+
+    expect_equal((f() %>% modelExtract),
+                 "a <- logit(x, 0, 10)")
+
+    f <- function() {
+      model({
+        a <- expit(x, high=10+1)
+      })
+    }
+
+    expect_equal((f() %>% modelExtract),
+                 "a <- expit(x, 0, 11)")
+
+    f <- function() {
+      model({
+        a <- expit(high=10+1, low=a, x)
+      })
+    }
+
+    expect_equal((f() %>% modelExtract),
+                 "a <- expit(x, a, 11)")
+
+
+  })
+
   test_that("logit tests", {
     expect_equal(
       rxToSE("logit(a)"),