Version 32 (modified by simonpj, 5 years ago) (diff)


This page discusses the design and potential implementation of "holes" in GHC. Discussion on this feature in GHC is in #5910. The development repository is here, and implementation issues are here.


Informally, a "hole" is something to be filled. A hole in the ground can cause an injury if you stepped into it, but you can also build a house around it without a problem. You can easily measure a hole to determine how much material (e.g. dirt or concrete) is needed to fill it.

The analogy of a hole in the ground can be transferred to a hole in a program. If you run the program and encounter a hole, the runtime will halt (e.g. as if you encountered undefined). But you can still compile a program with holes. The compiler reports information about the hole, so that the programmer knows what code should replace the hole.

These are the requirements of the problem that holes solve:

  1. Extract information about subterms in a program.
  2. Do not interrupt compilation.

The extracted information from a hole can include, among other things:

  • The expected type of the hole
  • The local bindings (and their types) in the scope of the hole

We first describe related work, including concepts that are similar in other languages as well as other approaches to solving the problem proposed. Then, we discuss the proposal in detail.

Related Work

Goals in Agda

One of the features of the Emacs mode for Agda is the ability to insert a goal, a placeholder for code that is yet to be written. By inserting a ? in an expression, the compiler will introduce a goal. After loading the file (which typechecks it), Agda gives an overview of the goals 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 the output

?0 : Bool
?1 : Bool

As can be seen here, goals are numbered, and the typechecker returns the inferred type for each of these goals.

Goals can make it a lot easier to write code. They allow typechecking to continue although certain parts of code are missing. They also work as a to-do list for incomplete programs.

Deferring type errors to runtime

The proposal DeferErrorsToRuntime implements a flag that turns every type error into a warning with an associated bit of code that is run when the offending ill-typed term is encountered at runtime. At the time of writing, this is implemented in GHC as -fdefer-type-errors.

Deferring type errors alone is not a solution that fits the problem description; however, it can be used in conjunction with other things to solve the problem.

Inserting deliberate type errors

One technique for finding the type of a subterm that has been often seen on the mailing lists is deliberately inserting ill-typed terms, so that the type error reports the expected type of the term.

Here is an example:

1 + ()

We get an error that there is no instance for Num ().

We can do this is a more refined manner by using undefined with a type annotation:

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

This gives the error:

    Couldn't match expected type `[Bool]' with actual type `()'
    In the first argument of `(++)', namely `(undefined :: ())'
    In the second argument of `(:)', namely `((undefined :: ()) ++ [])'
    In the expression: undefined : ((undefined :: ()) ++ [])
Failed, modules loaded: none.

The advantage of using undefined is that we can remove the type annotation, and the program will probably compile.

The clear problem with deliberate type errors is that the program does not type-check. We cannot use this technique on multiple locations at one time. We also only get type errors and not other information.

With deliberate errors and deferred type errors, we do get a program that type-checks. This is actually a reasonable solution; however, it still has two problems:

  • Deferring type errors is indiscriminate: you defer both the deliberate and unintended type errors.
  • It does not provide useful information other than type errors.

Implicit parameters

The implicit parameters extension allows a programmer to delay the binding of a variable while still preserving its type.

Here is an example:

$ ghci -XImplicitParams
Prelude> :t ?x : (?y ++ [])
?x : (?y ++ []) :: (?x::a, ?y::[a]) => [a]

The implicit parameters ?a and ?b appear as constraints in the type of the term containing them.

Implicit parameters must be bound, either in a term, e.g. let ?x = ... in ..., or in a type signature, e.g. let f :: (?x::a, Num a) => a; f = 1 + ?x. If an implicit parameter is not bound, it results in a type error. Of course, we can defer the type errors, but then we have the same problem with indiscriminate deferral.

Implicit parameters do not serve very well for debugging. Due to the binding requirement, they are not suitable for typing things with unknown types. Either you must change type signatures or you must deal with the unbound implicit parameter type errors (or warnings in case the errors are deferred). Lastly, since implicit parameters are meant for usage in programs, it does not seem like they should be used for extracting additional information about the parameter's location. (This is not a technical argument, of course.)


In this section, we discuss the proposed extension to GHC.

Language extension

Since we are changing the syntax and semantics of Haskell, we feel that this should become a language extension (rather than another kind of compiler flag). For now, we proposed the name Holes (as in -XHoles), though this could change after discussion.


We view a hole as a piece of syntax that is inserted in code as a placeholder until the programmer fills that location with something else. Numerous views on the syntax and semantics of holes have been discussed. Here are a few:

Term wildcards

This approach mirrors Agda goals. The actual syntax is debatable, but we think _ is quite nice, since it appears to be illegal as an expression.


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


  • Reports the source location of each hole
  • Does not allow two holes to have the same type SLPJ: This sounds wierd. Do you mean that _ && _ would be illegal somehow? Here the two holes both have type Bool
  • Requires evaluation semantics for hole. SLPJ: What does this mean? Example?

Named term variables

This approach mirrors implicit parameters. Each hole is given a name. Within a module (or even a program/library?), each every hole with the same name has the same type. Again, the actual syntax is debatable, but for now, we use _?x for a hole with the (possibly shared) name x.


test :: [Bool]
test = _?x : (_?y ++ [])


  • Reports the name and source location of each hole
  • Requires a fresh name to distinguish one hole from others
  • Requires evaluation semantics for hole

SLPJ question. I'm really not sure what you mean by "must have the same type. For example

f xs = _?p : xs
g ys = _?q : ys

If we typechecked each binding independently we'd get

f :: forall a. [a] -> [a]
g :: forlal b. [b] -> [b]

If we generalise f then NOTHING can have the same type as the hole. But it would be very strange not to generalise f (and g).

It seems much simpler to me either to have anonymous holes, or to let the user give them names, but to pay no attention to the name except to display them in error messages, so that user can say "oh, that hole" rather than having to look at the exact source location. End of SLPJ question

Term brackets (ranges)

Instead of an actual term, we can use a special form of bracketing to indicate a hole. As above, syntax is debatable, but for now, we use {_ and _} for the brackets of the hole.


test :: [Bool]
test = {_ undefined _} : ({_ undefined ++ [] _})


  • Reports the source location of each hole
  • Requires opening and closing brackets
  • Allows wildcard term to be treated as syntactic sugar, e.g. _ desugars into {_ undefined _}
  • Does not require evaluation semantics for the brackets

Note that we can extend this with names for each pair of brackets.

Type wildcards

Instead of term holes, we can use a special type to indicate an unknown type. The type hole would be reported. We use _ for the syntax here.


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


  • Reports the source location of each hole
  • Does not allow two holes to be equal
  • Does not require evaluation semantics for the holes
  • Can be used in type annotations for both variables and large expressions (as in the term brackets)
  • Allows partial types to be specified
  • (?)May not support reporting local bindings

Note that we can extend this with names for each type hole.

User's view

For this specification, we use the named term variables variant (though it may also apply to other variants).

When using holes (i.e. -XHoles is set), we expect the following:

  1. The program should type-check as if every hole _?h is replaced with undefined. There is an exception to this rule: see Ambiguous types below.
  2. If the program is well-typed (as above), then:
    • The types of all holes should be reported.
    • Reporting the hole types should not cause type-checking (or compiling in general) to stop (in error). SLPJ what does this mean? A type error *never* causes type checking to stop. Do you mean that a program with holes (but no other errors) should compile and run, falling over at runtime only if you evaluate a hole? Please give an example. End of SLPJ
  3. (optional) If the program is ill-typed, then:
    • The types of all holes should be reported.

Ambiguous types

Suppose that we replace every hole with undefined and type-check the program without -XHoles. Some programs would not type-check due to unsolved class constraints that result in ambiguous types. For example, show _?h becomes show undefined, and the Show constraint is not instantiated, so the program does not type-check.

We think holes can be extremely useful with ambiguous types. We prefer that a program, in which there is a hole with unsolved constraints on the type of the hole, still be considered well-typed, assuming the rest of the program is well-typed. In the above example, we would expect show _?h to have the type String with the reported hole type _?h :: Show a => a.

SLPJ do you want programs with these ambiguity errors to run too? Or what? Can you give a complete little example module, with the error messages you expect, whether you expect it to run, and if so what should happen?

Monomorphism restriction

Some ambiguous types fall under the monomorphism restriction. That is, the following program will not type under Haskell2010 due to the restriction that f have a monomorphic type:

f = undefined >>= undefined

We also expect holes to be very useful in these cases, for example by replacing each undefined with a hole:

f = _?h >>= _?i

Thus, we prefer that this program be considered well-typed with f :: Monad m => m b and the holes _?h :: Monad m => m a and _?i :: Monad m => a -> m b.

If -XNoMonomorphismRestriction is used, we expect that the typing of the holes will not change.

Type of a hole

The type of a hole should be the resolved type with minimum constraints. That is, the type of a hole should only have constraints that have not been solved but are either inferred from the context (e.g. show _?h) or given in a type annotation/signature (e.g. _?h :: Show a => a).