Using the %>% pipe, and dot (.) notation

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The problem isn’t map, but rather how the %>% pipe deals with the .. Consider the following examples (remember that / is a two argument function in R):

Simple piping:

1 %>% `/`(2)

Is equivalent to `/`(1, 2) or 1 / 2 and gives 0.5.

It is also equivalent to 1 %>% `/`(., 2).

Simple . use:

1 %>% `/`(2, .)

Is equivalent to `/`(2, 1) or 2 / 1 and gives 2.

You can see that 1 is no longer used as the first argument, but only as the second.

Other . use:

This doesn’t work however, when subsetting the .:

list(a = 1) %>% `/`(.$a, 2)

Error in `/`(., .$a, 2) : operator needs one or two arguments

We can see that . got injected twice, as the first argument and subsetted in the second argument. An expression like .$a is sometimes referred to as a nested function call (the $ function is used inside the / function, in this case).

We use braces to avoid first argument injection:

list(a = 1) %>% { `/`(.$a, 2) }

Gives 0.5 again.

Actual problem:

You are actually calling map(df, df$data, min), not map(df$data, min).

Solution:

Use braces:

df %>% { map(.$data, min) }

Also see the header Using the dot for secondary purposes in ?magrittr::`%>%` which reads:

In particular, if the placeholder is only used in a nested function
call, lhs will also be placed as the first argument! The reason for
this is that in most use-cases this produces the most readable code.
For example, iris %>% subset(1:nrow(.) %% 2 == 0) is equivalent to
iris %>% subset(., 1:nrow(.) %% 2 == 0) but slightly more compact. It
is possible to overrule this behavior by enclosing the rhs in braces.
For example, 1:10 %>% {c(min(.), max(.))} is equivalent to
c(min(1:10), max(1:10)).

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