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# Trees that Grow Guidance

The Trees that Grow (TTG) idiom can be used to provide different forms of extensions and variations on an AST. Since April 2018, the HsSyn AST inside GHC supports the TTG idiom. This page provides a set of guiding principles for GHC developers on how to understand and use the TTG idiom in HsSyn.

## Context and Scope

The new HsSyn AST supporting the TTG idiom (from now on referred to as TTG HsSyn) is designed to subsume five different representations of Haskell syntax:

- AST GhcPs: the AST used in GHC's parsing phase
- AST GhcRn: the AST used in GHC's renaming phase
- AST GhcTc: the AST used in GHC's typechecking phase
- AST TH: the AST used in Template Haskell
- AST HSE: the AST used in an external tool such as Haskell-Src-Exts

By "subsume" we mean that it should be possible to instantiate TTG HsSyn to serve all five use-cases.

The subsumption of above five ASTs is done by providing instances for the extension type families.
For instance, the AST for GHC's parsing, renaming, and typechecking are defined by providing instances of the extension type families using accordingly the indices `GhcPs`

, `GhcRn`

, and `GhcTc`

.
Here is the actual code providing such instances for the `HsExpr`

datatype of expressions in the TTG HsSyn.

## General pattern for TTG

In general a TTG-idiom data type has

- A type parameter, called the
*phase descriptor*, that indexes which particular instantiation is required - One
*extension field*in each data constructor, whose type is given by a type family. By giving phase-specific instances to this type family, we can add phase-specific information to the constructor. - One unary
*extension constructor*for each data type, whose argument type is given by a type family. By giving phase-specific instances to this type family, we can add extra phase-specific constructors to the type.

For example:

data Exp x = Var (XVar x) (IdP x) | Lam (XLam x) (IdP x) (Exp x) | App (XApp x) (Exp x) (Exp x) | New (XNew x) type family XVar x type family XLam x type family XApp x type family XNew x

Here the phase descriptor is `x`

. The first field of each constructor (of type `XVar x`

etc) are the extension fields. The data construcotr `XNew`

is the extension constructor.

All fields of the data constructors except the first (extension) field are called *payload fields*. They are present in every instantiation of the data type.

## Guiding Principles

The design of TTG HsSyn follows these principles:

- The base TTG HsSyn should have all the constructors common across all five ASTs (the
*common data constructors*). These constructors should have, as payload fields, all the fields common across all five ASTs.

- Note, however, that the type of a payload field of a constructor may vary with phase. For example, in
`Lam`

above, the first payload field has type`Id x`

, and that may vary with phase:type family IdP x type instance IdP GhcPs = RdrName type instance IdP GhcRn = Name type instance IdP GhcTc = Id

But it is still a payload field, because every instantiation of`Exp`

has a lambda with a binder; albeit the type of that binder field varies. This happens in HsSyn: for example, the type of the common (payload) field of the common constructor`HsVar`

of`HsExpr x`

is`IdP x`

where`IdP`

is a type family and`x`

the phase descriptor.

- The non-payload (i.e. phase-specific) fields of a data constructor are grouped together and introduced via the extension field. Similarly the phase-specific data constructors are introduced using the extension constructor.

- The instantiation of TTG HsSyn, for a particular phase, should result in a tree that has no redundant fields and constructors.

For example, the

`HsExpr GhsPs`

expressions of AST GhcPs should not have the constructor`HsUnboundVar`

of the post-renaming phases, or its`HsMultiIf`

constructor should also not have an unused field (of the type`Type`

) to store the related type produced in the typechecking phase.

As a result, the instantiated TTG HsSyn should not depend on the code from the other phases. Hence, the base (uninstantiated) TTG HsSyn should not depend on any GHC/TH/HSE-specific code.

For example, if

`HsExpr GhsPs`

expressions of AST GhcPs had the constructor`HsUnboundVar`

then it had to depend on the code defining`UnboundVar`

(a field of`HsUnboundVar`

) in the renaming phase, or if its constructor`MultiIf`

had a field of type`Type`

then it had to depend on the code defining`Type`

in the typechecking phase.

## Example

Consider the following three simple datatypes `ExpPs`

, `ExpRn`

, and `ExpTc`

representing correspondingly expressions in a parsing, renaming and typechecking phase:

module Parsing where -- the definition of RdrName -- ... data ExpPs = Var RdrName | Lam RdrName ExpPs | App ExpPs ExpPs

module Renaming where -- the definition of `Name` and `UnboundVar` -- ... data ExpRn = Var Name | Lam Name ExpRn | App ExpRn ExpRn | UVar UnboundVar

module Typechecking where -- the definition of `Id`, `UnboundVar`, and `Type` -- ... data ExpTc = Var Id | Lam Id ExpTc | App Type ExpTc ExpTc | UVar UnboundVar

Based on the TTG idiom, we will have a base declaration such as the following.

module AST where data Exp x = Var (XVar x) (IdP x) | Lam (XLam x) (IdP x) (Exp x) | App (XApp x) (Exp x) (Exp x) | New (XNew x) type family XVar x type family XLam x type family XApp x type family XNew x type family IdP x

and the following three instantiations:

module Parsing where import AST -- the definition of RdrName -- ... data Ps type ExpPs = Exp Ps type instance XVar Ps = () type instance XLam Ps = () type instance XApp Ps = () type instance XNew Ps = Void type instance IdP Ps = RdrName

module Renaming where import AST -- the definition of `Name` and `UnboundVar` -- ... data Rn type ExpRn = Exp Rn type instance XVar Rn = () type instance XLam Rn = () type instance XApp Rn = () type instance XNew Rn = UnboundVar type instance IdP Rn = Name

module Typechecking where import AST -- the definition of `Id`, `UnboundVar`, and `Type` -- ... data Tc type ExpTc = Exp Tc type instance XVar Tc = () type instance XLam Tc = () type instance XApp Tc = Type type instance XNew Tc = UnboundVar type instance IdP Tc = Id