 9 December 2021 — by Sam Derbyshire
Type-checking plugins, Part II: GHC's constraint solver

Type-checking plugins interface directly with GHC’s constraint solver. This means we need to have a certain level of familiarity with how GHC generates, manipulates and solves constraints.

In this post, we will attempt to demystify this constraint solving process, as well as review a few aspects of type-family reduction, to facilitate the development of type-checking plugins.

This post will serve as a reference, when we start writing our own type-checking plugins in Part III. Don’t worry: you don’t need to know everything before you start writing your own plugins! It’s only when debugging a type-checking plugin that the rubber hits the road: a few skills become rather important, such as the ability to understand the different types of constraints, to read Core (and, in particular, coercions), etc.

## Constraints and constraint solving

### Constraints

In Haskell, a constraint is a type whose values are determined implicitly through constraint solving, rather than passed explicitly as arguments.

#### Typeclass constraints

There are several different sorts of constraints. Let us consider first typeclass constraints, with the Eq typeclass as a first example.

type Eq :: Type -> Constraint
class Eq a where
(==), (/=) :: a -> a -> Bool

elem1 :: Eq a => a -> [a] -> Bool
elem1 _ []     = False
elem1 x (y:ys) = x == y || elem1 x ys
type EqDict :: Type -> Type
data EqDict a =
MkEqDict { eq, neq :: a -> a -> Bool }

elem2 :: EqDict a -> a -> [a] -> Bool
elem2 _      _ []     = False
elem2 eqDict x (y:ys) = eq eqDict x y || elem2 eqDict x ys

The difference between elem1 and elem2 is that in the latter, we pass the equality-checking function explicitly rather than implicitly. This distinction goes away in Core1, where we get:

elem1 :: forall a. Eq a => a -> [a] -> Bool
elem1 = \ @a ($dEq_a :: Eq a) (x :: a) (xs :: [a]) -> case xs of [] -> False (y:ys) -> (==) @a$dEq_a x y || elem1 @a $dEq_a x ys We are passing a record $dEq_a of type Eq a, and then using the field accessor

(==) :: forall a. Eq a -> (a -> a -> Bool)

to retrieve the method. We say that $dEq_a is a dictionary for the typeclass constraint Eq a; it provides evidence that the typeclass constraint is satisfied. This evidence is passed at runtime, as in the explicit dictionary-passing example.2 This means that defining a typeclass is tantamount to defining a record type, whose values will be obtained from constraint solving and passed implicitly (we will cover constraint solving in more detail later, see § Constraint solving). For instance, when calling elem1 (3 :: Int) [4,5], GHC will look up the instance Eq Int to obtain the dictionary, whereas the user would have to manually pass a record of type EqDict Int to use elem2 instead. #### Equality constraints GHC defines several different equality constraints that users can manipulate. These are: • Nominal equality, which can be homogeneous ~ or heterogeneous ~~,3 • Representational equality, Coercible (which is homogeneous). These all behave like ordinary typeclasses: evidence for such a constraint consists of a typeclass dictionary, as in the previous section. However, typeclass dictionaries in GHC are always boxed and lifted. In this case, this means that evidence for an equality is represented by a pointer, which might be an unevaluated thunk. This is rather unsatisfactory: we would prefer to only allow genuine equality proofs, as opposed to also allowing ⊥. In other words, we would like to rule out evidence of the form undefined or an error (e.g. a deferred type error, when -fdefer-type-errors is enabled). To solve this problem, GHC defines its own primitive equality types: • ~#, primitive nominal equality, and • ~R#, primitive representational equality. One can imagine that GHC internally defines: type (~) :: forall k. k -> k -> Constraint class a ~# b => a ~ b instance a ~# b => a ~ b type (~~) :: forall k l. k -> l -> Constraint class a ~# b => a ~~ b instance a ~# b => a ~~ b type Coercible :: forall k. k -> k -> Constraint class a ~R# b => Coercible a b instance a ~R# b => Coercible a b However, as ~# and ~R# are unlifted (and completely erased at runtime), we can’t define them as normal Haskell typeclasses as we attempted above. Instead, GHC defines these constraints internally, by specifying that the appropriate notion of evidence for an equality constraint is a coercion, a special part of the Core syntax. A value of type a ~# b is a genuine proof that the types a and b are equal – a nominal coercion. It can’t be undefined or an error. Similarly, a value of type a ~R# b is a genuine proof that the types a and b have the same runtime representation – a representational coercion. #### Coercions: a reading guide To support its type level features such as GADTs and type-families, GHC uses coercions to reason about equalities in types. This is most apparent with how Martin–Löf equality is encoded in a GADT: type (:~:) :: forall k. k -> k -> Type data a :~: b where Refl :: a :~: a Let’s see how GHC typechecks the following program: subst :: (a :~: b) -> a -> b subst Refl x = x We have x :: a, but we must produce a result of type b. We achieve this by casting x with the cast operator |>, using the evidence that a ~ b obtained by matching on Refl. subst = \ @a @b (eq :: a :~: b) (x :: a) -> case eq of Refl (co :: b ~# a) -> x |> ( Sub ( Sym co ) :: a ~#R b ) Notice how the Refl constructor has an extra argument in Core, here co :: b ~# a. We can think of Refl as simply boxing up a unboxed coercion. We can then cast x using the coercion co2 = Sub ( Sym co ) :: a ~#R b (to be explained below), obtaining ( a |> co2 ) :: b. GHC has a vast collection of coercions, which serves as its type-level proof language. It is important to be able to recognise some common coercions, as type-checking plugins will often manipulate them, and debugging a type-checker plugin often involves inspecting coercions. We provide here a short and non-exhaustive inventory of coercions one is liable to encounter in the depths of Core (refer to the GHC Core specification for a complete list). In the following, r denotes the role of the coercion: either N (Nominal) or R (Representational), with ~r denoting ~# or ~R#, respectively. • Reflexivity: <a>_r :: a ~r a. • Symmetry: we can reverse the orientation of co :: a ~r b to obtain Sym co :: b ~r a. • Transitivity: given co1 :: a ~r b and co2 :: b ~r c, we can compose them to get co1 ; co2 :: a ~r c. • Type constructor applications: we can apply a type constructor to coercions. For instance, given left_co :: a1 ~r a2 and right_co :: b1 ~r b2 we obtain (Either left_co right_co)_r :: Either a1 b1 ~r Either a2 b2. • Coercion applications: we can also apply one coercion to another. Given co_f :: f1 ~r f2 and co_arg :: arg1 ~# arg2 we get co_f co_arg :: f1 arg1 ~r f2 arg2. • Type constructor decompositions: we can decompose type constructor applications. For instance, given co :: Either a1 b1 ~# Either a2 b2, we can obtain Nth:0 co :: a1 ~# a2 and Nth:1 co :: b1 ~# b2. • Downgrading: given co :: a ~# b, we can downgrade its role to Representational, with Sub co :: a ~R# b. • Unsafe coercions: Univ r prov :: ty1 ~r ty2, where prov describes where such an universal coercion came from (its provenance), e.g. “a plugin unsafely claimed this equality”. • Coercion axioms, such as those derived from newtypes or type family equations; these are written AxiomName[i] for some natural number i; see below. Let us give a simple example of a coercion axiom; we will return to this subject later (see § Type family coercion axioms). When we define a newtype such as newtype Sum a = Sum { getSum :: a } we also obtain a way to coerce between Sum a and a for any type a. So we don’t have a single coercion, but rather a coercion constructor, or coercion axiom in GHC parlance. In this case, the coercion axiom is written Sum; it takes in a representational coercion co :: a ~#R b and returns a representational coercion Sum co :: Sum a ~#R b. #### Single-method dictionaries The dictionary associated with a typeclass with a single method is somewhat special, as it is defined as a newtype, not as a datatype. Thus, instead of retrieving dictionary fields, GHC coerces from the dictionary to the method using a cast. To illustrate, if we remove the (/=) method of Eq (so that Eq has a single method (==)), we get the following Core: elem1b :: forall a. Eq a => a -> [a] -> Bool elem1b = \ @a ($dEq_a :: Eq a) (x :: a) (xs :: [a]) ->
case xs of
[]     -> False
(y:ys) -> ( $dEq_a |> Eq <a>_N ) x y || elem1b @a$dEq_a x ys

This cast $dEq_a |> Eq <a>_N requires some explanation: we are casting the dictionary $dEq_a using the coercion Eq <a>_N; this is akin to using coerce in source Haskell. We can consider that GHC has internally defined:

newtype Eq a = MkEq { (==) :: a -> a -> Bool }

To go from $dEq_a :: Eq a to (==) :: a -> a -> Bool, we coerce. This means using a representational type equality. As we saw above, whenever we define a newtype, GHC creates a corresponding coercion axiom, in this case Eq, which takes a nominal coercion co :: a ~# b and returns a representational coercion Eq co :: Eq a ~R# (b -> b -> Bool). To retrieve the method (==) :: a -> a -> Bool, we apply the coercion axiom to the reflexive nominal coercion, written <a>_N. This results in the coercion above: Eq <a>_N :: Eq a ~R# (a -> a -> Bool) which we can then use to cast, obtaining the method: ($dEq_a |> Eq <a> ) :: a -> a -> Bool

### Constraint canonicalisation

Before reviewing how GHC goes about solving constraints, let’s first look at how GHC itself rewrites constraints and classifies them into different categories.

User-written constraints are born non-canonical: as GHC typechecks a type signature, it adds the constraints it comes across to its work list, to be processed later. Initially, GHC hasn’t analysed these constraints to determine their nature. For instance, one might have a type family

type TyFamCt :: Type -> Constraint
type family TyFamCt a where
TyFamCt Bool  = ( () :: Constraint )
TyFamCt (a,b) = a ~ b
TyFamCt c     = Integral c

In general, we don’t know what kind of constraint TyFamCt a is. If we can’t reduce the type family application, we’re stuck, so we say this is an irreducible constraint. However, we might be able to rewrite the type family application, e.g. TyFamCt Int will be canonicalised to Integral Int, a typeclass constraint; on the other hand, something like TyFamCt (x, y) will be canonicalised to x ~# y, an equality constraint.

Predicate Examples Evidence
Typeclass Ord a, Num a, (c1, c2), a ~ b Dictionary
Equality a ~# b, a ~R# b Coercion
Quantified forall a. Eq a => Eq (f a) Function
Irreducible c a, F x y Not yet known

The job of the canonicaliser is to rewrite the constraint as much as possible, e.g. reducing all type-family applications contained within the constraint. Note that, once a constraint is canonicalised, it is not necessarily frozen. For instance, TyFamCt x is an irreducible constraint, but we might later instantiate x to Int, in which case the constraint will be re-canonicalised into the typeclass constraint Integral Int.

### Constraint solving

#### A simple example

Let’s now briefly review how GHC goes about solving constraints. Consider the following simple example:

palindrome :: Eq a => [a] -> Bool
palindrome ds = ds == reverse ds

GHC compiles this program to the following Core:

palindrome :: Eq a => [a] -> Bool
palindrome = \ @a ($dEq_a :: Eq a) -> let$dEq_List_a :: Eq [a]
$dEq_List_a =$fEq_List @a $dEq_a in \ (ds :: [a]) -> (==) @[a]$dEq_List_a ds (reverse @a ds)

What has happened here is that the palindrome function was provided with the constraint Eq a, but in its body it calls (==) at type [a], which requires Eq [a].

In GHC parlance, we are solving an implication constraint, we have a Given constraint

[G] $dEq_a :: Eq a This means we have evidence for the constraint Eq a (which will be provided by the caller of the palindrome function). We must use it to synthesise evidence for the Wanted constraint [W]$dEq_List_a :: Eq [a]

GHC begins by canonicalising [G] $dEq_a :: Eq a; it’s obviously a dictionary constraint. This constraint then gets added to the inert set, which is the collection of constraints that GHC considers fully processed. As noted previously, addition of new information might allow rewriting to take place, in which case GHC could kick out a constraint (removing it from the inert set), to continue working on it later. Next, we canonicalise [W]$dEq_List_a :: Eq [a], which is also clearly a dictionary constraint. We then notice that the constraint Eq [a] matches with the class instance head

instance forall x. Eq x => Eq [x] where { .. }

(Recall that GHC only looks at instance heads when determining which instances to use; it never looks at the instance context before committing to an instance.)

Associated to the above instance is the dictionary function (or DFun)

$fEq_List :: forall x. Eq x -> Eq [x] which takes the dictionary evidence for Eq x and builds the corresponding dictionary evidence for Eq [x]. GHC thus solves the Wanted constraint: $dEq_List_a = $fEq_List @a$dEq_a

discharging it from the work list.

#### The interaction pipeline

To recapitulate, when processing a work list of constraints, GHC will pick a work item from the work list and take it through the interaction pipeline: In the “inert reactions” stage, the work item interacts with the constraints in the inert set. For instance, if the work item is a Wanted constraint, we might want to know whether we can solve it using Givens in the inert set (or perhaps just simplify it). In the “top-level reactions” stage, we use top-level instances. This is what happened when we used the top-level instance instance forall x. Eq x => Eq [x] to solve [W] Eq [a] using [G] Eq a.

Each step of this pipeline can change the work list or the inert set. After each step, we either

• go back to the start, e.g. because we want to change work item, or
• continue, with a possibly rewritten constraint.

Only once a work item makes it through the entire pipeline does GHC decide to then add it to the inert set.

#### The constraint solving loop

Now that we know how GHC processes its work list, we want to see how items get added to the work list in the first place. The most important aspect to understand is GHC’s constraint solving loop. Typically, when typechecking something, one can encounter two types of work:

• simple constraints, i.e. type-checking f (x :: a) = x + 1 will incur a simple Wanted Num a constraint,
• implication constraints, like we saw with the palindrome example, which gave rise to an implication a Given Eq a and a Wanted Eq [a].

The first step, that of simplifying simple Given constraints/solving simple Wanted constraints, is where type-checking plugins get to have their say: After processing simple constraints, GHC will proceed to go under implications. These can be nested: for instance, a successful pattern match on a GADT might introduce new information, which might need to be used when type-checking the RHS of the pattern match:

class C a where {..}
class D a where {..}
class E a where { methE :: a -> Int }

instance (C a, D a) => E a where {..}

data G a where
MkG1 :: C a => a -> G a
MkG2 :: Integral a => a -> a -> G a

foo :: D a => G a -> Int
foo x = case x of
MkG1 a -> methE a
MkG2 i j -> fromIntegral (i + j)

When type-checking foo, GHC processes the constraints, canonicalising them and eventually generating the following nested implications:

[G] D a
===>
[ [G] C a ===> [W] E a
, [G] Integral a ===> [W] Integral a, [W] Num a
]

To solve a collection of constraints, containing both simple Wanteds as well as implications, we proceed as follows:

• Run a constraint solving loop on all the simple Wanted constraints. Note that this step can create new implication constraints.
• Process the wanted implication constraints. For each implication, we will:
• Process its Givens (e.g. adding them to the inert set, when this makes sense).
• Solve the inner simple Wanted constraints.
• Recur into the nested implications.
• Reset the inert set to what it was before entering the implication, so that we can continue processing the other implications. #### Manually discharging single-method constraints

Note that GHC provides a mechanism to manually discharge single-method constraints, in the form of withDict (GHC 9.4 and above):

withDict :: c_dict -> (c => r) -> r

That is, one can discharge the c constraint by explicitly passing its dictionary, of type c_dict. For example:

class MyCt a where
myMeth :: a -> a -> a
-- NB: no instances.

myFun :: [Int] -> [Int] -> Int
myFun xs ys = withDict @(Int -> Int -> Int) @(MyCt Int) (+) (sum $zipWith myMeth xs ys) Here we have sum$ zipWith myMeth xs ys :: MyCt Int => Int, and we discharge the MyCt Int constraint by passing the dictionary (+) :: Int -> Int -> Int.

## Type families

Type families are one of the most powerful tools we have when type-level programming in Haskell, as they can be used to perform complex type-level reasoning. Let’s quickly review a few salient aspects, with a focus on the more powerful closed type families.

### A primer on type family reduction

For the most part, type family reduction follows the same general principles as regular pattern matching:

lookup :: forall k v. Eq k => k -> [(k,v)] -> Maybe v
lookup _ []
= Nothing
lookup k ( (l,v) : kvs )
| k == l
= Just v
| otherwise
= lookup k kvs

type Lookup :: forall k v. k -> [(k,v)] -> Maybe v
type family Lookup k kvs where
Lookup _ '[]
= Nothing
Lookup k ( '(k,v) : _ )
= Just v
Lookup k ( _ : kvs )
= Lookup k kvs

Already one difference can be spotted: type families support non-linear patterns, in which a variable occurs more than once. In this case, the second equation only matches if the first visible argument to Lookup, k, is equal to the first key in the list passed as second visible argument.

When reducing a type family application such as

Lookup "name" '[ '("occupation", "mathematician"), '("name", "晴三") ]

we step through to check which branch to take. In this case:

• the first branch doesn’t match: the second (visible) argument is not an empty list,
• the second branch doesn’t match: "name" is definitely not equal to "occupation",
• the third branch does match, so we reduce:
Lookup "name" '[ '("occupation", "mathematician"), '("name", "晴三") ]
~
Lookup "name" '[ '("name", "晴三") ]

We can keep going: this time the second equation matches, and we reduce further:

Lookup "name" '[ '("name", "晴三") ] ~ "晴三"

Another peculiar aspect of type family reduction is that the forall quantifier in the kind signature is actually a misnomer: in reality, type families behave as if we had a relevant (instead of irrelevant) dependent quantifier. This simply means that we are allowed to perform case analysis on the quantified kind variables, for instance:

type Weird :: forall k. k
type family Weird where
Weird = Int
Weird = Maybe

weird :: Weird -> Weird Int
weird x = Just (x+1)

What is happening here is that the type family is implicitly matching on the invisible argument:

type family Weird @k where
Weird @Type           = Int
Weird @(Type -> Type) = Maybe

weird :: Weird @Type -> Weird @(Type -> Type) Int

That is, we should really be writing:

type Weird :: foreach k. k

Here foreach denotes relevant dependent quantification (i.e. a dependent product type), and is one of the Dependent Haskell quantifiers. The use of this quantifier corresponds to the fact that implicitly quantified kinds behave as normal arguments. Indeed, when writing a type-checking plugin, we will see that they are put on the same footing as visible arguments.

### Type family coercion axioms

We’ve just seen some basic aspects of type-family reduction in Haskell. When writing a type-checking plugin, it’s also important to know about the implementation of type families in GHC. As type families come from equations between types, it should come as no surprise that they are closely tied with coercions.

It’s never a bad idea to look at Core to see how GHC handles things. Let’s take the following simple example:

type F :: Type -> Type
type family F a where
F Word      = Int
F (Maybe a) = Maybe (F a)
F a         = a

g :: Maybe (F Word) -> F (Maybe Int)
g mb_x = fmap negate mb_x

We obtain the following Core (omitting role information and downgrades):

g1 :: [F Word] -> [Int]
g1 = \ ( mb_x :: Maybe (F Word) ) ->
fmap @Maybe $fFunctorMaybe @Int @Int ( negate @Int$fNumInt )
( mb_x |> Maybe F )

g :: [F Word] -> F [Int]
g = g1 |> ( <Maybe (F Word)> -> Maybe ( Sym ( F <Int> ) ) ; Sym ( F <Int> ) )

Recalling the syntax of coercions from § Coercions, a reading guide, we can make sense of this. The basic principle is that GHC associates to F a coercion axiom with three branches:

• F :: F Word ~# Int,
• F, which takes in co :: a ~# b and returns F co :: F (Maybe a) ~# Maybe (F b),
• F, which takes in co :: a ~# b and returns F co :: F a ~# b.

Now, looking at g1 first:

• In g1, we use the 0-th equation of F to obtain the coercion F :: F Word ~# Int. We then apply the Maybe type constructor to F to obtain Maybe F :: Maybe (F Word) ~# Maybe Int. This allows us to typecheck \ mb_x -> fmap negate ( mb_x |> Maybe F ) at the type Maybe (F Word) -> Maybe Int.

• The second piece of the puzzle comes from the following two coercions:

• F <Int> :: F (Maybe Int) ~# Maybe (F Int),
• F <Int> :: F Int ~# Int.

As we need to rewrite from Maybe Int to F (Maybe Int), we reverse these using Sym, and apply the Maybe type constructor to the second coercion so that we can compose them:

• Maybe ( Sym ( F <Int> ) ) ; Sym ( F <Int> ) :: Maybe Int ~# F (Maybe Int).

As we are casting between function types, we need to handle the argument type as well as the result type. We use the coercion we just obtained for the result type; for the argument type, the reflexive coercion <Maybe (F Word)> will do.

We thus recover the coercion that we saw in the Core:

<Maybe (F Word)> -> Maybe ( Sym ( F <Int> ) ) ; Sym ( F <Int> )
:: ( Maybe (F Word) -> Maybe Int ) ~# ( Maybe (F Word) -> F (Maybe Int) )

### Conclusion

We’ve now seen:

• how GHC represents constraints, which will be useful when we want to manipulate them in a plugin,
• how GHC solves constraints, and at which point plugins get invoked during constraint solving,
• some aspects of type family reduction and their relation with coercions, which will be important to know when rewriting type family applications in a plugin, and in debugging Core Lint issues.

In the next part of this series, we will see how to write a type-checking plugin. You certainly don’t need to know everything outlined here to get started, but you can refer back to this document in case something goes wrong (e.g. constraint solving not behaving as expected, or a Core Lint error).

1. Core is a small functional language, based on System F with added coercions, which Haskell is desugared into after typechecking. Many of GHC’s optimisations happen on the level of Core. To see the Core that GHC generates when compiling a Haskell program, use the -ddump-simpl GHC option; additional flags such as -dsuppress-uniques -dsuppress-idinfo -dsuppress-ticks -dsuppress-module-prefixes can make the Core easier to read.
2. This evidence must be passed at runtime: the types are erased, so we must use another mechanism to record which instance was selected (i.e. which implementation of the typeclass methods to use).
3. A homogeneous coercion is one that relates types of the same kind, while a heterogeneous coercion can relate two types of different kinds.  This article is licensed under a Creative Commons Attribution 4.0 International license.