rmonad
: Where’s the monad?This work is funded by the National Science Foundation grant NSF-IOS 1546858.
You probably don’t need to read this. Certainly you should read the introduction
vignette first.
** This vignette is under construction **
This vignette consists of four parts. First I will describe the monad hidden in the R runtime. Second, I will describe how rmonad
can serve as a replacement. Third, I will contrast the monadic pipelines of rmonad
with the compositional pipelines of magrittr
. Finally, I will discuss rmonad
from the Haskell perspective.
I will introduce the concept of a monad incrementally through the first three sections. However, monads in the programming context are notoriously difficult to understand. If you are not familiar with them, you may try studying a few online tutorials first. That said, rmonad
can be used without understanding monads.
rmonad
The goal of rmonad
is to ditch the existing impure R monad and replace it with a clean explicit monad.
x %>>% sum %>>% sqrt
The initial %>>% operators acts as both a return
and bind
function. It first evaluates the
m b
is dependent on m a
, not just on a
. The bind
function can pass information from one step in the pipeline m a
to the next m b
.
m2 b
is equal to m3 b
only for the trivial case where the context is identity
.
R users normally rely on the R session to automatically perform these binds.
But what exactly is m
? In an R session, the R runtime handles errors. If one function raises an error, the error is propagated to functions that use its input.
In rmonad
, the m
is an object, that catches all undefined behavior.
rmonad
versus magrittrTo understand the monadic nature of rmonad
it is useful to compare it to magrittr
. In magrittr
, the expression x %>% foo %>% bar
is the same as bar(foo(x))
. From a monadic point of view, x
is first implicitly raised into an ‘Identity’ monad (which is quite formless here) m1 x
. Then
bind :: m1 x -> (x -> m2 y) -> m3 y
The bind
function above executes foo
on x
, yielding m2 y
. It then reduces this to m3 y
in the presence of m1
. But magrittr
passes no information from m1
to m3
. The pipeline is context indepent.
In rmonad
, the pipeline x %>>% foo %>>% bar
will pass a record of past events at each bind operation, thus incrementally building a graph of the pipeline.
rmonad
for HaskellersThe name rmonad
is a nod to Xmonad
(no relation to the Restricted Monad package). Where Xmonad
wraps the X window system, rmonad
wraps evaluation of R expressions.
Only a few R expressions are pure. If a function is given an invalid input at runtime (e.g. sqrt("wtf")
), it will die with a message printed to stderr. rmonad
wraps all R calls in a monad, intercepting all messages, so that the result of a computation is returned as a pure object.
The ‘R monad’ is one monad to rule the all. There is no monad stack and no support for monad transformers. In addition to error handling, The monad stores the history of every previous operation. It also performs basic benchmarking, recording the time required for each operation and the size of the returned object. All this weight might seem like a performance killer, but R programmers are used to function calls being slow, so if they care about performance, they wouldn’t use a function in a tight loop anyways.
The return
function is a little complex in rmonad
. It is a special case of the as_monad
function:
as_monad = a -> m b
Where a
can be one of three types
an unevaluated R expression - as_monad
evaluates the expression, capturing any exceptions, warnings or messages. as_monad
is used inside the bind
function in this capacity.
a pure R value - acts as return
The %>>%
operator is like the Haskell >>=
operator, but with some of the sloppiness expected of a dynamic language:
%>>% :: m a -> (a -> m b) -> m b
| a -> (a -> m b) -> m b
| (a -> r b) -> (a -> m b) -> m b
%>>%
differs from >>=
in that %>>%
automatically loads the left-hand-side value into a monad.