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README.md

elm-compiler-docs

This guide is assembled by the Elm community, with no assistance from Elm's creator. It's a reverse engineering effort in progress. It's based on the 0.18 compiler. If you're new to Haskell or would appreciate a refresher, see haskell-resources.md for some helpful resources and tutorials.

Getting Started

Haskell and Cabal can be a pain to set up, especially so for the Elm compiler 0.18, which uses an earlier version of Haskell. The easiest way to get the codebase running on your computer is through Elm Env, a Docker image that sets up a contained Haskell environment in a container.

Once you have Elm Env up and running, navigate to Elm-Platform/0.18. From here you can build the compiler from source using the following commands:

$ cabal install elm-compiler
$ cabal install elm-make

These commands will build the compiler codebase. Note that, in order to keep the latest build in your path (inside the Docker container), you may have to run:

$ export PATH=$PATH:/Elm-Package/Elm-Platform/0.18/.cabal-sandbox/bin

Alternatively the executables can be found at /Elm-Package/Elm-Platform/0.18/.cabal-sandbox/bin.

elm-make

Elm-make is the command-line tool that manages and invokes the elm-compiler when it is processing a single file. The codebase is much smaller and less complex than elm-compiler, and as such it provides a handy entry point to understanding the Elm compiler in depth.

There is a handy file in the elm-make source (the command-line tool that runs elm-compiler on a .elm file), called TheMasterPlan.hs, that models each step in the build process, annotating the types of each intermediate representation. Evan has provided some helpful comments along the way that point to how each intermediate representation is generated, and what it is used for. The elm-make codebase provides the code for all the parts of 'the compiler' that we are familiar with--the CLI entry point, dependency crawling and aggregation, and the files that are generated as a result of the build.

You can trace the points at which it enters the Elm compiler codebase in Compile.hs. I would recommend thoroughly wrapping your head around the data types and build process in elm-make before deep-diving into the elm-compiler code itself.

How the compiler works

The entry point of the compiler lists various passes over the source. (If you're new to compilers, read this.)

  • Parse the source code
  • Canonicalize all variables, pinning down where they came from
  • Run type inference
  • Nitpick (miscellaneous checks)
  • Optimization
  • Code generation

The manager processes that run these phases can be found in the Elm directory. The entry point of the compiler is Compiler.hs.

The compile process happen inside the Result type. This is like Elm's Result type but on steroids, with lots of places to put information about errors. It's also declared as a monad, which for our purposes makes it work well with chained operations (i.e. bail out if there are any errors), and allows the use of do notation. If this doesn't make sense to you, see haskell-resources for a refresher on monads.

Result is one of many tools defined under Reporting which are used to manage errors. A Report represents some kind of error that gets printed when your program fails to compile.

A Region describes the place in the code where the error happened; other types can be bundled with Region using Located a defined Reporting/Annotation.hs. The kinds of errors are descibed in Reporting/Error.hs which farms them out to submodules: Canonicalize, Docs, Pattern, Syntax, and Type. Errors can be rendered to human-readable text or to JSON (by --format=json but that might not actually work?).

Error detection starts by examining small pieces of code (parsing characters, duplicate record fields), expands out to larger ones (name and type conflicts within and across modules), and then focuses back in on specific things (the type of main, exhaustive pattern matches, documentation).

Evan's 0.18 release greatly improved Elm's error reporting, and this is really one of the outstanding aspects of the Elm compiler: the compiler is very good at providing meaningful errors. See Syntax.hs for some examples of the helpful error messages Elm provides.

AST

The Abstract Syntax Tree is the main intermediate representation of code. It is used throughout the stages of compilation.

Expressions

A fully general expression has four type variables describing different pieces of information. An Expr' is a giant union type of possible Elm expressions, such as Literal, Binop, Cmd, OutgoingPort, etc. Expr is this, but with a top-level annotation that contains useful information about where the expression came from, and other debugging information. There are Source, Valid and Canonical versions of expressions, which represent the expressions at different stages of the compiler lifecycle: Source expressions are created by the parser, and when the compiler is complete they have been resolved to Canonical expressions.

These versions are type aliases that provide specific types for the type variables. Optimized expressions, apparently because they need less information, are a separate union type.

Variables

Variable.hs shows how variables are represented in the AST, and provides some utility functions for both general and inbuilt variables.

Types

Elm's type system is relatively simple (which is to say, not nearly as complex as Haskell's). Type.hs provides definitions for Raw and Canonical types, where canonical means the same as it does in the case of expressions; that the type representation is fully annotated.

Declarations

A Declaration is anything that can be at the top level within a module: a definition, a union type, a type alias, a port, an infix declaration. There is also code for patterns, variables, literals, and types.

Parse

Parsing is the first stage of compilation, and is built around the Parsec library. Parsing is organized by parsers for expressions, declarations, literals, types, etc. The IParser a type is a parser that attempts to parse a string into an a (think JSON decoders). The parser's job is to transform valid code into the AST, and to detect and provide helpful error messages for invalid code.

The parser entry point validates (Validate.hs) the declarations for syntax errors (not including parse errors). Such errors include type annotations missing definitions, ports without annotations, duplicate report field names, and too many/too few type variables. Validation also ensures that ports do not occur outside of the main module; the isRoot parameter refers to the root module (i.e. Main), not the root user.

Canonicalize

Canonicalization enriches the AST with more information in preparation for type inference. It determines what is visible where, and ensures there are no scoping problems. Canonicalization also sorts declarations by dependency.

Type Inference

There's no shortage of academic papers on type inference, and what Elm has is relatively basic, but still quite complex. Type inference is a constraint-solving algorithm that works by unifying constraints to produce the most general possible type given the constraints.

(I don't really know what's going on but Evan had some tidbits here: https://github.com/elm-lang/elm-compiler/issues/1281)

Nitpick

Nitpick is a collection of mostly-unrelated checks that happen after type inference. Nitpicking verifies the type of main, adds warnings for missing type annotations, and (I think?) flags inexhaustive pattern matches.

Generate

Code generation traverses the AST and outputs JavaScript, with help from the Language.ECMAScript3 Haskell package. The code is triggered from Compiler.hs, entering the Generate directory through the generate function in JavaScript.hs. Much of the generate process occurs in elm-make, including the generation of .elmi and .elmo files, crawling dependencies, and the provision of boiler JS code necessary for the JS runtime.

The generated JS is a combination of two different definition sets, the defsList (Elm code generated from libraries and source), and the managerStmts (effects managers).

defsList

The list of definitions is generated the the generateDef function. Essentially what this does is delegate to a big case statement that transforms the list of Elm expressions into JS expressions, in Haskell types, through the ECMAScript 3 package. These expressions are then 'printed' to stmtsToText function in Builder.hs. Most of the hard work in this translation is contained in the ECMAScript definitions, which allow the Elm compiler to meaningfully represent a JS program in Haskell types.

The JS code that is generated is then padded by some boilerplate JS in elm-make, which does all the work of setting up the connections between the browser and the code generated by the Elm compiler. It is elm-make that generates elm-stuff and other build artifacts associated with the actual CLI interface.

managerStmts

To come: what exactly are effects managers? This tutorial on effects is incomplete, which makes it hard to tell exactly.