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Functions are first-class in R. Define with function(args) body:

square <- function(x) {
    x * x
}

print(square(5))    # 25

The last expression is the return value — return() is optional.

Named arguments and defaults:

greet <- function(name, greeting = "Hello") {
    cat(greeting, ", ", name, "!\n", sep="")
}

greet("Ada")                       # Hello, Ada!
greet("Ada", greeting = "Hi")      # Hi, Ada!
greet(name = "Ada")                # Hello, Ada!

R uses lazy evaluation — arguments aren't evaluated until used. This means you can pass expressions that would error if always evaluated:

default_or <- function(x, fallback) if (is.null(x)) fallback else x
default_or(NULL, stop("unreachable"))   # safe — fallback isn't evaluated