wip

Mailund_article_promises_env.R

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+
+```{r Mailund}# {{{
+Idea:  Treat separately an R expression and the env to evaluate it.
+R can NOT manipulate a promise.  See Ch 20, quosures to turn promise into R object.  Before get to Hadley, MAILUND seems clearer
+https://mailund.dk/posts/promises-and-lazy-evaluation/
+pryr::  note spelling
+```
+```{r mailund2, echo=TRUE}
+# why does this work?
+# b/c evaluation is function frame, not calling frame
+
+f <- function(x, y, z = 2 * x) x + z
+a <- b <- 1
+f(a + b, stop("ARGH!!!"))
+```
+```{r}
+f,g created in Global Env.
+create `promises` only; no evaluation
+Case I, f is also called from Global env
+Case II, f is called when current_env is NOT Global
+```
+
+```{r}
+library(rlang)
+# create in Global
+f <- function(x) pryr::promise_info(x)
+
+# Case I
+# call f in Global
+f(x)   # function env is Global
+
+# Case II
+# call f inside a differnt env
+g <- function(z) {
+	print(current_env())
+	print(fn_env(f)) # Global
+  f(z)
+}
+g(x + y)
+
+# f,g
+env_names(current_env())
+```
+
+```{r}
+# promise = expression, env (where called)
+# Watch, once evaluate x, promise's  env disappears
+# Also, before evaluation, current_env not same as promise env
+# Mailmund cont
+
+```