Embedding effects in JavaScript using Monads
Last updated 3 years ago by effjs .
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JavaScript embedded effects compiler

This is a JavaScript to JavaScript transpiler. It offers extending JavaScript language with various effects by means of runtime libraries, without even using compiler plugins.

There are such libraries for:

Not yet implemented:

  • probabilistic programming
  • parallel and distributed programming
  • persistent continuations
  • adaptive computations

Theare are typically small, some of them are just tiny wrappers of well known interfaces, such as Promises and Rx Observables.

The compiler converts ES5 to ES5 without any syntax extensions. So it may be applied to results of other compilers targeting JS such as CoffeeScript, TypeScript, Babel etc.

It stratifies input JavaScript code into two levels, namely object and meta level. Their separation may be either explicit or implicit.

Generators syntax can be used for explicit level separation. This way following code:

function* () {
  console.log("x:",yield getX());

will be translated into:

function() {
  return getX().mapply(function (b) { console.log("x:", b); });

Or with implicit mode input code may be even more succinct:

function() {

The output will be the same.

There are more examples of input/output in the test folder.

The mapply function there is abstract. For example its concrete implementation for promises is their then function. There is a dozen of such functions required to be implemented in concrete effects implementation library. There is a library with default implementations of most of them using small basis. The interface builds on Monads interfaces hierarchy from Haskell (Functor, Applicative, Alternative, Monad).

It is arguable if explicit or implicit levels separation is better. This likely depend on what kind effect is used. The succinct and more readable code is good, but if effects are heavy making them explicit may be better. So effectfuljs compiler supports both options.

I will abuse term pure for some JS code or values. This doesn't mean the code is really pure of course. This is original JavaScript and it is absolutely not a problem to use the side effects already embedded in JavaScript. Including IO, references, exceptions, etc.

Besides two primary explicit and implicit modes, there are means to treat some parts of the code selectively to be either effectful or pure.


There are quite a few JavaScript transpilers adding some concrete new effects into JavaScript language. This tool embeds abstract side effects into language. Any concrete effect is a runtime library implementing that abstract interface.

One of the examples is recent ES standard updates with generators and async/await. It is a new concrete side effect embedded into language. Adding same coroutines effects with effectful-js doesn't require standard, syntax change and new compilers.

Human readability for generated code aim is shared with kneden transpiler, and turning async/await into promises expressions. The same may be achieved using effectful-js with a tiny adapter from promises interface into effectfuljs. There is one implemented in @mfjs/promise, so effectfuljs approach does require runtime library loading, while kneden team highlights no runtime library dependency as an advantage. There is a plan to implement combinators inlining in effectfuljs, so generated code for promises library will be very similar. Also, effectfuljs is more complete than kneden(at the time of writing this). It at least can handle breaks/return/continue from try/catch/finally.

There are other less known JS extensions may be implemented as library using effectfuljs compiler. These are webppl for probabilistic programming, flapjax language for reactive programming.

A few other JS libraries abstract generators interface to any monad, for example burrido. It works pretty well if effects don't require re-executing of some control paths several times. Which is the case for reactive, logical programming and continuations. Here is a problem description for rx monad with burrido and this is the same with effectfuljs.

In other languages the most famous examples of similar tools are Haskell do-notation and C# LINQ. They implement explicit separation of meta and object levels. They have different syntaxes. In JS burrido does the same. The effectful level expressions are generators expressions, with interface adapted to arbitrary monad, while pure code parts use plain JS syntax.

In single level syntax the layers separation is implicit, and both use the same language. The first mention of this I know is embedding monads using delimited control by Andrzej Filinski in Representing monad paper from 1994. There is a more recent implementation of the same idea for Java in quasar framework described in this post. Continuations based implementation doesn't allow detecting and automatically generating Applicative combinators instead of Monadic ones, for more efficient code (more details in Applicative vs Monad interface).

There are also concrete side effects compilers with single level syntax, for example flapjax and webppl.


The current version is a library on top of estransducers JS transformation framework. You won't typically use it directly unless you develop own effects library. There is a babel preset for default translations @effectfuljs/babel-presetp-env, or some effects library may provide similar preset.

The preset is an extension of babel-preset-env and accepts the same arguments with additional effjs object specifying options for effectfuljs framework.

First install it:

$ npm install --save-dev babel-preset-env @effectfuljs/babel-preset-env

Next compile your sources with whatever babel tool you prefer using @mfjs/env preset instead of babel env, for example in .babelrc:

  "presets": ["@effjs/env"]

Code transformation

By default it doesn't touch any code, so it is safe to apply it to all JS files in the project. The translation is activated when it encounters CommonJS require for core library module. By default it is @mfjs/core but may be configured. The require call expression must be assigned to some namespace variable and that namespace name is used to detect translation directives further in the code. I will use name M for core namespace in the doc.

In default mode (after require) it still doesn't translate anything. To start actual translation specify how eager you want it to be with M.profile directive receiving a string with the name of the mode. For example "defaultFull" mode may be used to treat all function calls as effectful in all function definition except the current one.

In short, the tool converts effectful monadic values in code into its inner pure value. For example for promises it converts Promise object into the value it is going to be resolved to (or already resolved). In generated code this is converted to appropriate then usages. The backward translation is also needed. In the promises example the code may require access to original promise object, for example to wait for a few promises in parallel.

From pure to effectul

  • M.reify - expects function expression, executes it immediately but doesn't translate value to pure one.

From effectful to pure

The general backward translation from effectful value to pure value can be performed using M directive. That will translate monadic value into pure even in "minimal" profile. In "full" profile all functions call will be immediately reflected into pure value.


Since JavaScript is dynamically typed language we cannot know in compile type if the function will indeed return monadic value or it may return any pure value or no value at all. For this to work the library must keep checking the type of the value and construct monadic value (with M.pure function) if it is not already monadic. This of course adds additional overhead, so there is an option to disable coercions. In this case generated code will always return effectful values from effectful functions, and will not add runtime checks. This, of course, requires stronger discipline for functions not translated with the compiler using some strong code style conventions or probably some future type checker. If accidently not effectful functions are called it will crash.

There is also exceptions coercion. If it is enabled (disabled by default) all function invocation will be wrapped into try..catch block and in the case of exception its value is translated into M.raise.

The coercion level is defined using coerce option with possible values: "none", "all" or "value".

Variables scope

Some monads may re-execute some parts of control path several times. The most typical example is logical programming monad, reactive programming and continuations.

Programmers would expect variables values to revert their values on backtracking in a typical logical programming language. The compiler tries to capture some local variables values for this to work. Not local variables are global references (the embedded into JavaScript references effects). If you apply some mutable operator to a local variable it will also be visible on backtracking. This is a good reason to move to immutable data structures though.

To avoid local variables capturing use M.ref directive specifying variables as arguments. If the code will be used only for monads where variable capturing is not needed (like Promise) it may be disabled to avoid overhead, by setting option varCapt to false.

Applicative vs Monad interface

There is interfaces hierarchy Functor <- Applicative <- _Monad. Functor allows only changing its inner value of effectful value. Applicative allows combining several effectful values into one, and Monad is the most generic one allows changing structure of effectful value depending on inner value. In effectful the main corresponding functions for the interfaces are mapply, M.arr, mbind respectively.

Looking at differences between mapply and mbind, the first one always returns pure value while the second (with enabled coercion) may either return pure or effectful value. And in fact if coercions is active if mbind returns pure value it is semantically equivalent to mapply. So one may wonder why mapply is needed at all. Monad generalizes Applicative interface, so if something has Monad interface it is automatically Functor and Applicative. But monadic expressions are not suitable for static analysis.

For example, for parser combinators monad this means Monad-based one allows introducing context dependencies. Depending on some already parsed part it may return grammar for the next part. While Applicative combinators don't offer such context dependency but they allow building much more efficient parsers. It may analyze grammars during parser construction (calculate FOLLOW, FIRST sets etc). That is impossible for Monad parser because parts of its grammar will be known only during parsing.

Applicative interface allows implicit program parallelization like in haxl.

For some functional reactive libraries which build a data-flow graph, the monadish application means graph rebuilding (switching). The same applicative version will have that graph always static. So for example if-then-else, if this is implemented as Monad the two branches graph will always be rebuilt on new value of condition, and there will be only one branch constructed. While for Applicative-based one both branches will be always built and only signal propagation will continue to some single branch depending on input value. This may be more efficient for libraries where graph construction is expensive, because of some possible optimizations.

At the present version compiler always tries to translate expressions into Applicative form, unless they are logical operators (&&, ||) or conditional (?:), or they change some variable value using direct assignment or update (+=, ++, =). This diverges from JavaScript semantics because order of operations and whence side effects order isn’t defined. This is a bad code practice to have such operation in a single expression anyway. But you may still disable this by setting option expr: "seq".

The compiler will translate for-loop into M.forPar if its tests and update expressions are pure, tests and body don’t change any variable (assignment is allowed). Some monad implementation may run each iteration in parallel.

Another option is to translate a sequence of statements into parallel blocks. It will use M.par function in generated code. It takes an array of monadic values and by default returns another monadic value representing a sequence of computations. A concrete monad's implementation is free to override it to something more efficient. The compiler may optionally try to reorder computations to get more such parallel blocks.

The translation is similar to applicative do notation for Haskell described in the paper: Desugaring Haskell’s do-notation Into Applicative Operations. To enable it in some block use parBlock option with following possible values:

  • all - grouping all statements in par block, regardless its possible dependencies
  • byUsage - groups statements into single par block if they don't have shared variable.
  • byLhsUsage - same like byUsage but will avoid grouping if some variable is updated before used.
  • reorderByUsage - like byUsage but reorders statement to get bigger parallel blocks
  • reorderByLhsUsage - like byUsage but reorders statements too

In the future versions the compiler will try to translate more code patterns into Applicative form.


Many monads may support multiply inner values in single monadic value. These are reactive programming, logical programming monads etc.

If monad supports this you may use either method from the interface directly or directives (M.answer or M.yield), yield expression (if not in "regenerator" profile). All three are aliases and have the same encoding. It acts similar to return statement but allows continuing same function executions after the point where they were invoked adding more answers to result of the function.

No answer result is M.empty function call. Yield will discharge no answer values in the current block (between {}). So execution will be not performed in single block between something executing M.empty and up till next and including M.yield.


Top level fields of the options object:

  • ns - name of namespace variable used for importing core library using ES6 modules or CommonJS require, it may be specified explicitly if no imports is used
  • packageName - name of core library package used to detect CommonJS require call to guess packageVar in sources (default is @mfjs/core)
  • profile - initial mode name name, disabled by default
  • verbose - debug output


There is a reference implementation library @mfjs/core with documentation for each function, but it is not required to use that library. It is default but other libraries may provide other encodings.

The generated code expects monadic value to have following methods:

  • mbind
    • takes function argument and applies it to inner value of this returning coerced monadic result (Haskell’s >>= function from Monad class)
  • mapply - takes function argument and applies it to inner value of this replacing that inner value with result without changing monadic value structure (Haskell fmap function from Functor class)
  • mfinally - takes a function and executes after control exits this block
  • mhandle
    • takes a function and executes it if this throws exception

And the imported core library should have following free functions:

  • M - coerces value, if returns argument as-is if it is already monadic or M.pure(v) otherwise.
  • M.pure - returns monadic value with inner value from argument and with no effects
  • M.raise - returns monadic value representing exception throw
  • M.repeat - takes a function and initial arguments for it, apply that function infinitely, threading output arguments to input of next function invocation
  • M.block - takes a function and executes it giving another function as argument for exiting the block, it is used for break, continue, yield encodings
  • M.scope - same as M.block but for the whole function, for return statement encoding
  • M.generator - same as M.scope but may be injected for generator functions if enabled. Its callback function has 3 arguments, for return, yield and yield* resp.
  • M.arr - takes array of monadic value and returns monadic value of array of inner values corresponding to input array, this is from Haskell’s Applicative interface, but adapted for more convenient usage in JavaScript

If state is enabled:

  • M.set
  • M.modify
  • M.get

TODO: more details

The compiler requires specific iterator interface. It is not compatible with ES iterators because they are mutable, while for some monads execution control may backtrack to some already passed position. In fact if ES allowed such iterators cloning this compiler wouldn't be needed.

An iterator is a function object, with value field for current value.

  • M.iterator - takes ES iterable object returns effectfuljs compatible iterator, it is just interface adapter, but works like ES one, taking next iterator invalidates previous ones, returned iterator already points to first element or null if input collection is empty
  • M.forInIterator - returns effectfuljs compatible iterator for for-in statement

Selective transform

Because of coercions it is pretty ok to transform just everything with "full" profile but overhead of some heavy monads is quite sensible. There are means to apply transformation option to some parts of code. There are predefined profiles mentioned before but there is also flexible tuning utility. You may specify some very custom project policy based on some code conventions in your project.


The directive performs some named action to change internal state of transformations. There are a few predefined states.

  • "defaultMinimal" - in following function definitions doesn’t translate anything except it is specified as exception in configuration (for example M function).
  • "defaultFull" - in next function definitions treats as effectful all function calls except exceptions from configuration (for example by default there window, process, and console functions are exceptions)
  • "full" - same as "defaultFull" but starts immediately
  • "minimal" - same as "defaultMinimal" but starts immediately
  • "generatorsDo" - very similar to minimal mode but uses generators syntax extensions, it will compile without coercions all function*, and in them treat yield expression as M.refect and yield* as M in minimal mode.
  • "generators" - will convert generator functions to use M.generator as its scope

The scope of these predefined profiles is a block in curly brackets.


The function simply merges the object from argument into current option object. Its scope is a block in curly bracket by default. When translation exits the block where M.option was called it will revert states object to the one used before entering function. The interpretation of the fields' value in the object depends on exact passes implementation.


The toolset doesn't introduce any syntax extension, it uses a set of predefined function as directives to provide some translation options. They are executed at compile time. Here is the list of currently used ones:

  • M/M.reflect - converts from effectful expression to pure
  • M.reify - same as M.p but receives function expression which is to be called immediately
  • M.option - changes current options object
  • M.profile - changes current state


Copyright © 2016-2017 Vitaliy Akimov

Distributed under the terms of The MIT License (MIT).

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