cond-thread: Conditionally evaluate expressions and thread them

In alekrutkowski/clojR: Clojure-inspired functions (macros) in R

Description

See https://clojuredocs.org/clojure.core/cond->. NOTE: This R version is more powerful (general) than the Clojure version as it is threading the obj value not only through the right-side expresions but also the conditions/tests (while in Clojure these are static conditions).

Usage

"cond->"(obj, ...)

Arguments

obj	An object/value.
...	An even number of expressions. A set of test/expression pairs.

Examples

# Adapted from the first example at https://clojuredocs.org/clojure.core/cond->

`cond->`(1,               # we start with 1
         `==`(1), `+`(1), # 1==1 is true so 1+1 is evaluated and yields 2 which is threaded further
         `<`(0), `*`(42), # 2<0 is false so the operation is skipped
         `==`(2), `*`(3)) # 2==2 is true so 2*3 is evaluated and it finally yields 6

# A version closer in spirit to the Clojure example -- the constants need to be wrapped
# in anonymous functions or in a function which ignores its first argument
# (see `constant` below):

`cond->`(1,                          # we start with 1
         function(x) TRUE, `+`(1),   # the condition is true so 1+1 yields 2
         function(x) FALSE, `*`(42), # the condition is false so the operation is skipped
         function(x) 2==2, `*`(3))   # 2==2 so it yields 6

constant <- function(ignore_me, v) v

`cond->`(1,                        # we start with 1
         constant(TRUE), `+`(1),   # the condition is true so 1+1 yields 2
         constant(FALSE), `*`(42), # the condition is false so the operation is skipped
         constant(2==2), `*`(3))   # 2==2 so it yields 6

alekrutkowski/clojR documentation built on May 11, 2019, 11:24 p.m.