Version 2 (modified by xnyhps, 6 years ago) (diff)

Used a bit better example, updated info on implicit parameters


One of the features of the Emacs mode for Agda is the ability to add goals. By inserting a ? in an expression, the compiler will introduce a hole. After loading the file (which typechecks it), Agda gives an overview of the holes in the file and their types.

For example:

test : List Bool
test = Cons ? (Cons ? Nil)

Gets turned into:

test : List Bool
test = Cons { }0 (Cons { }1 Nil)

With extra output:

?0 : Bool
?1 : Bool

As can be seen here, holes are numbered, and the typechecker returns the name for each of these holes.

These goals can be useful as placeholders when writing code. They allow typechecking to continue although certain parts of code are missing and they make a good TODO list.


GHC does not support holes in the way Agda does. It is possible to insert undefined in an expression to make it typecheck (which Agda doesn't have), but this is not very helpful when writing software. Inserting undefined only gives as information that the rest of the program typechecks, but will not help you find what you needed to use in its place. We propose to add an extension to GHC (and notably GHCi) to allow using holes, this page is meant to discuss the exact features and workflow of such an extension.

First, two existing features that can be used as holes.


As stated before, undefined typechecks just like a hole: it has type a, so it can be used anywhere. However, it is not very easy to use in this way: it is impossible to find out what type the compiler found for the hole, and it's impossible to get a list of all the holes used in your source file(s).

The same example:

test :: [Bool]
test = undefined : (undefined ++ [])

Will not help finding the types of the undefineds at all. One advantage is that the code will not refuse to run, unless one of the undefineds is actually evaluated.

Implicit Parameters

The GHC extension Implicit Parameters comes closer to how we'd expect holes to work. It makes it possible to specify a term with a question mark, denoting a implicit variable.

Same example:

test = ?a : (?b ++ [])

Inspecting the type of test when defined in GHCi now shows the types of the (unbound) implicit parameters:

> :t let test = ?a : (?b ++ []) in test :: [Bool]
let test = ?a : (?b ++ []) in test :: [Bool]
  :: (?a::Bool, ?b::[Bool]) => [Bool]

However, defining test like this in a module gives the following error:

    Unbound implicit parameter (?a::Bool)
      arising from a use of implicit parameter `?a'
    In the first argument of `(:)', namely `?a'
    In the expression: ?a : (?b ++ [])
    In an equation for `test': test = ?a : (?b ++ [])

    Unbound implicit parameter (?b::[Bool])
      arising from a use of implicit parameter `?b'
    In the first argument of `(++)', namely `?b'
    In the second argument of `(:)', namely `(?b ++ [])'
    In the expression: ?a : (?b ++ [])
Failed, modules loaded: none.

This will show you the type, however, it does consider it an error and fails, so there may be other problems you don't get to see because of it. It also will refuse to load and compile the module, so it's impossible to run the parts of it that are finished.

The reason is that the hole becomes a part of the type signature, as a constraint. So to correctly use it here, the function would have to be written as:

test :: (?a::Bool, ?b::[Bool]) => [Bool]
test = ?a : (?b ++ [])

This makes it very impractical to use them as holes, as all type signatures have to be updated to let the typechecker continue. Not only in the functions that use the implicit parameter itself, but they propagate upwards, just like class constraints: if another function were to call test, it would have the same implicit parameters (and therefore, all of these type signatures would have to be updated when a new hole is added). Another tricky problem with implicit parameters is that implicit parameters with the same name in different functions are not assumed to be the same parameter (i.e., required to be unifiable), except if some function has both implicit parameters in its constraints. Lastly, it's impossible to run code with unbound implicit parameters, even if the parameters are never actually used.

Now for possible ways we see holes working in GHC.


The simplest way would be to implement them in the same way as Agda: add a new syntax (we shall use two underscores as an example here, __) to denote a hole, and after typechecking, show the user a list of all the types of all the holes in their source files. In what cases "after typechecking" we do this is still subject of discussion. We expect at the very least to show it after (re)loading a module into GHCi or typechecking an expression in GHCi directly (:t).


test :: [Bool]
test = __ : __ : []

Theoretical output:

> :l test.hs
[1 of 1] Compiling Main             ( test.hs, interpreted )
Found a hole at test.hs:2:6-7: Bool
Found a hole at test.hs:2:11-12: Bool

Named holes

Implicit parameters have some good features too: they can be named, and so used in multiple locations. In the Agda-style, this would require let-binding a hole, which is a lot of effort for something that should be a temporary placeholder. So one idea is to allow giving holes a name, just like implicit parameters.

For example:

test :: [Bool]
test = _a : _b : []

test2 = (_a, _b)

Theoretical output:

> :l test.hs
[1 of 1] Compiling Main             ( test.hs, interpreted )
Found a hole _a: Bool
Found a hole _b: Bool

These could either be made shared within a module, or not (so not the confusing situation with implicit parameters, that are only shared when required).

Not holes, ranges

Holes can be useful for finding the type of something that still needs to be written, but it's also possible one might want to figure out the type of a part of an existing expression. For example, a user could be trying to understand a big, complicated function he didn't write himself, and trying to figure out what the types are of certain parts of that function could involve a lot of looking up of other functions.