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layout developer-doc
title Monadic Contexts
category types
tags
types
context
monad
effect
order 8

Monadic Contexts

Coming from a Haskell background, we have found that Monads provide a great abstraction with which to reason about program behaviour, but they have some severe usability issues. The main one of these is the lack of automatic lifting, requiring users to explicitly lift computations through their monad transformer stack.

For a language as focused on usability as Enso is this really isn't feasible. To that end, we have created the notion of a 'Monadic Context', which is a monad transformer based on Supermonads (see references). These have special support in the compiler, and hence can be automatically lifted to aid usability.

The actionables for this section are:

  • Think about subsumption for contexts.
  • Contexts (e.g. IO) are represented using T in IO. Multiple contexts are combined as standard (IO | State Int), and it is written the same in arg position.
  • Do we definitely want to use monads, or can we use arrows or other interpreter-based effects systems? These may aid with parallelism analysis.

Context Syntax

There are three main notes about the syntax of contexts:

  1. Monadic contexts are defined using the in keyword (e.g. Int in IO).
  2. We have a symbol !, which is short-hand for putting something into the Exception monadic context. This is related to broken values.
  3. Contexts can be combined by using the standard typeset operators, or nested through repeated uses of in.

Monadic Bind

It is also important to note that Enso has no equivalent to <- in Haskell. Instead, pure computations are implicitly placed in the Pure monadic context, and = acts to 'peel off' the outermost layer of contexts. As such, this means that = always acts as bind, greatly simplifying how the type-checker has to work.

Context Definitions

Contexts can be defined by users.

The actionables for this section are:

  • How, what, when and why?

Context Lifting

The actionables for this section are:

  • Specify and explain how automated lifting of monadic contexts works.
  • It depends on the order of runCtx

Inbuilt Contexts

Enso includes a set of commonly-used monadic contexts as part of Base, its standard library. These are listed below.

The actionables for this section are:

  • Determine the full set of contexts that Enso should provide by default.

IO

The actionables for this section are:

  • Determine the granularity of IO (it's not one context, but a lot).
  • Explain how there is no runIO, and IO is just run at the program boundary, as well as the impacts of this.

State

The actionables for this section are:

  • Determine exactly how state works (the fact that the 'keys' are preset by the runState)
  • Describe how dependently-typed maps allow us to provide more flexible interfaces in future.