Elaborator Reflection

The Idris elaborator is responsible for converting high-level Idris code into the core language. It is implemented as a kind of embedded tactic language in Haskell, where tactic scripts are written in an elaboration monad that provides error handling and a proof state. For details, see Edwin Brady’s 2013 paper in the Journal of Functional Programming.

Elaborator reflection makes the elaboration type as well as a selection of its tactics available to Idris code. This means that metaprograms written in Idris can have complete control over the elaboration process, generating arbitrary code, and they have access to all of the facilities available in the elaborator, such as higher-order unification, type checking, and emitting auxiliary definitions.


Elaborator reflection is a new, experimental feature. It will change. You’ve been warned.

The Elaborator State

The elaborator state contains information about the ongoing elaboration process. In particular, it contains a goal type, which is to be filled by an under-construction proof term. The proof term can contain holes, each of which has a scope in which it is valid and a type. Some holes may additionally contain guesses, which can be substituted in the scope of the hole. The holes are tracked in a hole queue, and one of them is focused. In addition to the goal type, proof term, and holes, the elaborator state contains a collection of unsolved unification problems that can affect elaboration.

The elaborator state is not directly available to Idris programs. Instead, it is modified through the use of tactics, which are operations that affect the elaborator state. A tactic that returns a value of type a, potentially modifying the elaborator state, has type Elab a. The default tactics are all in the namespace Language.Reflection.Elab.Tactics.

Running Elaborator Scripts

On their own, tactics have no effect. The meta-operation %runElab script runs script in the current elaboration context. For example, the following script constructs the identity function at type Nat:

idNat : Nat -> Nat
idNat = %runElab (do intro (Just (UN "x"))
                     fill (Var (UN "x"))

On the right-hand side, the Idris elaborator has the goal Nat -> Nat. When it encounters the %runElab directive, it fulfills this goal by running the provided script. The first tactic, intro, constructs a lambda that binds the name x. The name argument is optional because a default name can be taken from the function type. Now, the proof term is of the form \x : Nat => {hole}. The second tactic, fill, fills this hole with a guess, giving the term \x : Nat => {hole≈x}. Finally, the solve tactic instantiates the guess, giving the result \x : Nat => x.

Because elaborator scripts are ordinary Idris expressions, it is also possible to use them in multiple contexts. Note that there is nothing Nat-specific about the above script. We can generate identity functions at any concrete type using the same script:

mkId : Elab ()
mkId = do intro (Just (UN "x"))
          fill (Var (UN "x"))

idNat : Nat -> Nat
idNat = %runElab mkId

idUnit : () -> ()
idUnit = %runElab mkId

idString : String -> String
idString = %runElab mkId

Interactively Building Elab Scripts

To build an Elab script interactively, use the :elab command at the REPL. It takes the name of a hole as an argument. The interface is largely the same as in Idris’s interactive prover, except all meta-commands start with colon. For example, use :qed instead of qed, :abandon instead of abandon, and :undo instead of undo.


Some tactics may fail. For example, intro will fail if the focused hole does not have a function type, solve will fail if the current hole does not contain a guess, and fill will fail if the term to be filled in has the wrong type. Scripts can also fail explicitly using the fail tactic.

To account for failure, there is an Alternative instance for Elab. The <|> operator first tries the script to its left. If that script fails, any changes that it made to the state are undone and the right argument is executed. If the first argument succeeds, then the second argument is not executed.

Querying the Elaboration State

Elab includes operations to query the elaboration state, allowing scripts to use information about their environment to steer the elaboration process. The ordinary Idris bind syntax can be used to propagate this information. For example, a tactic that solves the current goal when it is the unit type might look like this:

triv : Elab ()
triv = do compute
          g <- getGoal
          case (snd g) of
            `(() : Type) => do fill `(() : ())
            otherGoal => fail [ TermPart otherGoal
                              , TextPart "is not trivial"

The tactic compute normalises the type of its goal with respect to the current context. While not strictly necessary, this allows triv to be used in contexts where the triviality of the goal is not immediately apparent. Then, getGoal is used, and its result is bound to g. Because it returns a pair consisting of the current goal’s name and type, we case-split on its second projection. If the goal type turns out to have been the unit type, we fill using the unit constructor and solve the goal. Otherwise, we fail with an error message informing the user that the current goal is not trivial.

Additionally, the elaboration state can be dumped into an error message with the debug tactic. A variant, debugMessage, allows arbitrary messages to be included with the state, allowing for a kind of “printf debuggging” of elaboration scripts. The message format used by debugMessage is the same for errors produced by the error reflection mechanism, allowing the re-use of the Idris pretty-printer when rendering messages.

Changing the Global Context

Elab scripts can modify the global context during execution. Just as the Idris elaborator produces auxiliary definitions to implement features such as where-blocks and case expressions, user elaboration scripts may need to define functions. Furthermore, this allows Elab reflection to be used to implement features such as type class deriving. The operations declareType, defineFunction, and addInstance allow Elab scripts to modify the global context.

Using Idris’s Features

The Idris compiler has a number of ways to automate the construction of terms. On its own, the Elab state and its interactions with the unifier allow implicits to be solved using unification. Additional operations use further features of Idris. In particular, resolveTC solves the current goal using type class resolution, search invokes the proof search mechanism, and sourceLocation finds the context in the original file at which the elaboration script is invoked.

Recursive Elaboration

The elaboration mechanism can be invoked recursively using the runElab tactic. This tactic takes a goal type and an elaboration script as arguments and runs the script in a fresh lexical evironment to create an inhabitant of the provided goal type. This is primarily useful for code generation, particularly for generating pattern-matching clauses, where variable scope needs to be one that isn’t the present local context.

Learn More

While this documentation is still incomplete, elaboration reflection works in Idris today. As you wait for the completion of the documentation, the list of built-in tactics can be obtained using the :browse command in an Idris REPL or the corresponding feature in one of the graphical IDE clients to explore the Language.Reflection.Elab.Tactics namespace. All of the built-in tactics contain documentation strings.