# Instance Arguments¶

Instance arguments are the Agda equivalent of Haskell type class constraints and can be used for many of the same purposes. In Agda terms, they are implicit arguments that get solved by a special instance resolution  algorithm, rather than by the unification algorithm used for normal implicit arguments. In principle, an instance argument is resolved, if a unique instance of the required type can be built from declared instances  and the current context.

## Usage¶

Instance arguments are enclosed in double curly braces `{{ }}`, e.g. `{{x : T}}`. Alternatively they can be enclosed, with proper spacing, e.g. `⦃ x : T ⦄`, in the unicode braces `⦃ ⦄` (`U+2983` and `U+2984`, which can be typed as `\{{` and `\}}` in the Emacs mode).

For instance, given a function `_==_`

```_==_ : {A : Set} {{eqA : Eq A}} → A → A → Bool
```

for some suitable type `Eq`, you might define

```elem : {A : Set} {{eqA : Eq A}} → A → List A → Bool
elem x (y ∷ xs) = x == y || elem x xs
elem x []       = false
```

Here the instance argument to `_==_` is solved by the corresponding argument to `elem`. Just like ordinary implicit arguments, instance arguments can be given explicitly. The above definition is equivalent to

```elem : {A : Set} {{eqA : Eq A}} → A → List A → Bool
elem {{eqA}} x (y ∷ xs) = _==_ {{eqA}} x y || elem {{eqA}} x xs
elem         x []       = false
```

A very useful function that exploits this is the function `it` which lets you apply instance resolution to solve an arbitrary goal:

```it : ∀ {a} {A : Set a} {{_ : A}} → A
it {{x}} = x
```

Note that instance arguments in types are always named, but the name can be `_`:

```_==_ : {A : Set} → {{Eq A}} → A → A → Bool    -- INVALID
```
```_==_ : {A : Set} {{_ : Eq A}} → A → A → Bool  -- VALID
```

### Defining type classes¶

The type of an instance argument should have the form `{Γ} → C vs`, where `C` is a postulated name, a bound variable, or the name of a data or record type, and `{Γ}` denotes an arbitrary number of (ordinary) implicit arguments (see Dependent instances below for an example where `Γ` is non-empty). Instance arguments that do not have this form are currently accepted, but instance resolution may or may not work as described below for such arguments.

Other than that there are no requirements on the type of an instance argument. In particular, there is no special declaration to say that a type is a “type class”. Instead, Haskell-style type classes are usually defined as record types. For instance,

```record Monoid {a} (A : Set a) : Set a where
field
mempty : A
_<>_   : A → A → A
```

In order to make the fields of the record available as functions taking instance arguments you can use the special module application

```open Monoid {{...}} public
```

This will bring into scope

```mempty : ∀ {a} {A : Set a} {{_ : Monoid A}} → A
_<>_   : ∀ {a} {A : Set a} {{_ : Monoid A}} → A → A → A
```

Superclass dependencies can be implemented using Instance fields.

See Module application and Record modules for details about how the module application is desugared. If defined by hand, `mempty` would be

```mempty : ∀ {a} {A : Set a} {{_ : Monoid A}} → A
mempty {{mon}} = Monoid.mempty mon
```

Although record types are a natural fit for Haskell-style type classes, you can use instance arguments with data types to good effect. See the Examples below.

### Declaring instances¶

As seen above, instance arguments in the context are available when solving instance arguments, but you also need to be able to define top-level instances for concrete types. This is done using the `instance` keyword, which starts a block in which each definition is marked as an instance available for instance resolution. For example, an instance `Monoid (List A)` can be defined as

```instance
ListMonoid : ∀ {a} {A : Set a} → Monoid (List A)
ListMonoid = record { mempty = []; _<>_ = _++_ }
```

Or equivalently, using copatterns:

```instance
ListMonoid : ∀ {a} {A : Set a} → Monoid (List A)
mempty {{ListMonoid}} = []
_<>_   {{ListMonoid}} xs ys = xs ++ ys
```

Top-level instances must target a named type (`Monoid` in this case), and cannot be declared for types in the context.

You can define local instances in let-expressions in the same way as a top-level instance. For example:

```mconcat : ∀ {a} {A : Set a} {{_ : Monoid A}} → List A → A
mconcat [] = mempty
mconcat (x ∷ xs) = x <> mconcat xs

sum : List Nat → Nat
sum xs =
let instance
NatMonoid : Monoid Nat
NatMonoid = record { mempty = 0; _<>_ = _+_ }
in mconcat xs
```

Instances can have instance arguments themselves, which will be filled in recursively during instance resolution. For instance,

```record Eq {a} (A : Set a) : Set a where
field
_==_ : A → A → Bool

open Eq {{...}} public

instance
eqList : ∀ {a} {A : Set a} {{_ : Eq A}} → Eq (List A)
_==_ {{eqList}} []       []       = true
_==_ {{eqList}} (x ∷ xs) (y ∷ ys) = x == y && xs == ys
_==_ {{eqList}} _        _        = false

eqNat : Eq Nat
_==_ {{eqNat}} = natEquals

ex : Bool
ex = (1 ∷ 2 ∷ 3 ∷ []) == (1 ∷ 2 ∷ []) -- false
```

Note the two calls to `_==_` in the right-hand side of the second clause. The first uses the `Eq A` instance and the second uses a recursive call to `eqList`. In the example `ex`, instance resolution, needing a value of type ```Eq (List Nat)```, will try to use the `eqList` instance and find that it needs an instance argument of type `Eq Nat`, it will then solve that with `eqNat` and return the solution `eqList {{eqNat}}`.

Note

At the moment there is no termination check on instances, so it is possible to construct non-sensical instances like `loop : ∀ {a} {A : Set a} {{_ : Eq A}} → Eq A`. To prevent looping in cases like this, the search depth of instance search is limited, and once the maximum depth is reached, a type error will be thrown. You can set the maximum depth using the `--instance-search-depth` flag.

### Examples¶

#### Dependent instances¶

Consider a variant on the `Eq` class where the equality function produces a proof in the case the arguments are equal:

```record Eq {a} (A : Set a) : Set a where
field
_==_ : (x y : A) → Maybe (x ≡ y)

open Eq {{...}} public
```

A simple boolean-valued equality function is problematic for types with dependencies, like the Σ-type

```data Σ {a b} (A : Set a) (B : A → Set b) : Set (a ⊔ b) where
_,_ : (x : A) → B x → Σ A B
```

since given two pairs `x , y` and `x₁ , y₁`, the types of the second components `y` and `y₁` can be completely different and not admit an equality test. Only when `x` and `x₁` are really equal can we hope to compare `y` and `y₁`. Having the equality function return a proof means that we are guaranteed that when `x` and `x₁` compare equal, they really are equal, and comparing `y` and `y₁` makes sense.

An `Eq` instance for `Σ` can be defined as follows:

```instance
eqΣ : ∀ {a b} {A : Set a} {B : A → Set b} {{_ : Eq A}} {{_ : ∀ {x} → Eq (B x)}} → Eq (Σ A B)
_==_ {{eqΣ}} (x , y) (x₁ , y₁) with x == x₁
_==_ {{eqΣ}} (x , y) (x₁ , y₁)    | nothing = nothing
_==_ {{eqΣ}} (x , y) (.x , y₁)    | just refl with y == y₁
_==_ {{eqΣ}} (x , y) (.x , y₁)    | just refl    | nothing   = nothing
_==_ {{eqΣ}} (x , y) (.x , .y)    | just refl    | just refl = just refl
```

Note that the instance argument for `B` states that there should be an `Eq` instance for `B x`, for any `x : A`. The argument `x` must be implicit, indicating that it needs to be inferred by unification whenever the `B` instance is used. See Instance resolution below for more details.

## Instance resolution¶

Given a goal that should be solved using instance resolution we proceed in the following four stages:

Verify the goal

First we check that the goal is not already solved. This can happen if there are unification constraints determining the value, or if it is of singleton record type and thus solved by eta-expansion.

Next we check that the goal type has the right shape to be solved by instance resolution. It should be of the form `{Γ} → C vs`, where the target type `C` is a variable from the context or the name of a data or record type, and `{Γ}` denotes a telescope of implicit arguments. If this is not the case instance resolution fails with an error message[1].

Find candidates
In the second stage we compute a set of candidates. Let-bound variables and top-level definitions in scope are candidates if they are defined in an `instance` block. Lambda-bound variables, i.e. variables bound in lambdas, function types, left-hand sides, or module parameters, are candidates if they are bound as instance arguments using `{{ }}`. Only candidates that compute something of type `C us`, where `C` is the target type computed in the previous stage, are considered.
Check the candidates

We attempt to use each candidate in turn to build an instance of the goal type `{Γ} → C vs`. First we extend the current context by `Γ`. Then, given a candidate `c : Δ → A` we generate fresh metavariables `αs : Δ` for the arguments of `c`, with ordinary metavariables for implicit arguments, and instance metavariables, solved by a recursive call to instance resolution, for explicit arguments and instance arguments.

Next we unify `A[Δ := αs]` with `C vs` and apply instance resolution to the instance metavariables in `αs`. Both unification and instance resolution have three possible outcomes: yes, no, or maybe. In case we get a no answer from any of them, the current candidate is discarded, otherwise we return the potential solution `λ {Γ} → c αs`.

Compute the result

From the previous stage we get a list of potential solutions. If the list is empty we fail with an error saying that no instance for `C vs` could be found (no). If there is a single solution we use it to solve the goal (yes), and if there are multiple solutions we check if they are all equal. If they are, we solve the goal with one of them (yes), but if they are not, we postpone instance resolution (maybe), hoping that some of the maybes will turn into nos once we know more about the involved metavariables.

If there are left-over instance problems at the end of type checking, the corresponding metavariables are printed in the Emacs status buffer together with their types and source location. The candidates that gave rise to potential solutions can be printed with the show constraints command (`C-c C-=`).

 [1] Instance goal verification is buggy at the moment. See issue #1322.