Monad implementation in Julia
Author tk3369
9 Stars
Updated Last
1 Year Ago
Started In
October 2019


Build Status

This is an experimental package with functions that works with the following types of monads:

  • Maybe
  • Either / Result
  • List



The fmap function can map over any Maybe monad (either Just or None). If the input is wrapped as a Just object, the output is automatically wrapped as well. NONE is a singleton constant of None.

Unlike other implementations, a fmap'ed function can also take ordinary values rather than monads. It would then apply the function as usual and return an ordinary result. So the result is not elevated to a wrapped monad.

1       |> fmap(x -> x + 1)   # 2
just(1) |> fmap(x -> x + 1)   # Just(2)
NONE    |> fmap(x -> x + 1)   # NONE

Use or_else to switch over to a useful value when NONE is encountered.

1        |> or_else(2)        # 1
just(1)  |> or_else(2)        # Just(1)
NONE     |> or_else(2)        # 2

Use cata to execute either left function when the value is nothing or the right function when the value is something useful.

1        |> cata(() -> 0, x -> x + 1)     # 2
NONE     |> cata(() -> 0, x -> x + 1)     # 0

It is possible to extend to your own Just and None types by implementing the MaybeTypeTrait. Note that Nothing is given a IsNone trait by default.


The Either type is used to capture either a left or right object. To create an Either object, simply use the left or right function. Use left_value orright_value to extract the wrapped value. Use is_left or is_right to check if an object is left or right. There is no discrimination which way is better.

A special case of Either is Result, which is used for exception handling. Use the result constructor to create a Result object. By default, any subtypes of ErrorException are considered left. Everything else is considered right.

julia> result(1)

julia> result(ArgumentError("bad input"))
MonadResult_Error(ArgumentError("bad input"))

The convenient is_left and is_right functions can be used to check if the object is left or right. To extract value from the object, use left_value or right_value.

julia> is_right(result(1))

julia> is_left(result(ArgumentError("bad input")))

julia> right_value(result(1))

julia> left_value(result(1))
ERROR: MethodError: no method matching left_value(::Either{:R,:Result})


A List monad is essentially a 1-dimensional array. Use the list constructor to create a new list monad. We can fmap over all elements, or flatten a nested list.

julia> m = list(1)
1-element Array{Int64,1}:

julia> v = list([1,2,3])
3-element Array{Int64,1}:

julia> fmap(x -> 2x, v)
3-element Array{Int64,1}:

julia> flatten([1, [2,3], [[4],[5]]])
5-element Array{Int64,1}:

More Examples

Using maybe monad to handle Nothing

Maybe is a monad that either contains something useful or nothing. How is it useful? Sometimes certain functions returns nothing rather than throwing exception to indicate a negative condition For example:

match(r"^a.*", "hello")     # nothing

It is a bit unfortunate that we must test the condition before using the result:

matched = match(r"^a.*", "hello")
result = if matched !== nothing
    matched.match * " world"

If we have the notion of Maybe, then we can do it in a functional style:

"hello" |> match(r"^a.*") |> matched |> concat(" world")

To make that happen, we can do the following to create composable functions that only take single arguments.

Base.match(re::Regex) = Base.Fix1(match, re)
matched(rm::RegexMatch) = rm.match
concat(s::String) = Base.Fix2(string, s)

If you don't like type piracy then define your own match function or convince the Julia core developers that it is a good addition to the Base library. And, this would work just fine:

julia> "hello" |> match(r"^h.*") |> matched |> concat(" world")
"hello world"

That's close but this doesn't work for the nothing condition.

julia> "abc" |> match(r"^h.*") |> matched |> concat(" world")
ERROR: MethodError: no method matching matched(::Nothing)

With the help of fmap function, we can make it work:

julia> "abc" |> fmap(match(r"^h.*")) |> fmap(matched) |> fmap(concat(" world")) === nothing

This is getting a little long and hard to read, so we just compose the functions:

process = fmap(
    concat(" world")

using Test
@test process("hello") == "hello world"
@test process("abc") == nothing

Look ma, it is just a data flow pipeline without any conditional statement.

Using result monad for exception handling

Either is a monad that contains data on the left side or right side. It is useful to keep track of two scenarios. For examples:

julia> going_to_party = left("I am sick")
MonadEither_Left(I am sick)

julia> is_left(going_to_party)

julia> play_badminton = right("this weekend")
MonadEither_Right(this weekend)

julia> is_right(play_badminton)

julia> right_value(play_badminton)
"this weekend"

Result is a monad that is a special case of Either. By convention, we stay on the right track for normal conditions but switch to the left track when we encounter an error condition. Once we're on the left track, we stay on the it and ignore all computation until the end. As the error condition was captured when we switch to the left track, we can tell what went wrong when we come out of the computation. As you can see, Either monad is useful in handling errors.

A simple example is to run a database query. As part of the process, we need to establish a connection, obtain a database cursor, and then run the query. The trouble is that it may throw an exception at any of the database api calls:

    conn = get_connection(url)
    cursor = get_cursor(conn)
    sql = "select * from somehwere"
    return query(cursor, sql)
catch ex 
    @error "Unable to run query due to ex=$ex"

It would be nice if the error just flows to the end. Without using try-catch statement, we would like to do this:

# anonymous function to make it composable
run_query(sql) = cursor -> query(cursor, sql)

# error handler
handle_query_result(err::LeftEither) = @error(left_value(err))

# result set handler
handle_query_result(rs::DataFrame) = "good job" 

# establish pipeline
result = fmap(
    run_query("select * from sometable"),

The returned result from run_query is either a good value or an error. We can find out if it's good or bad by calling is_right and is_left respectively. If needed, we can also dispatch based upon ResultEither or ErrorEither types.