buzden / idris2-if-unsolved-implicit

An interface helping to set a value to an implicit when it is unsolved

If your implicit is unsolved

Little helper to have a default value for an implicit which otherwise would be unsolved

Small artificial example

Consider the following datatype

data X : Nat -> Type where
  MkX : (m : Nat) -> X n

As you can see, its constructor has an implicit parameter which can be determined by its return type:

x2 : X 2
x2 = MkX 16

Sometimes a polymorphic value can be swallowed by a polymorphic function, e.g.:

unXm : X n -> Nat
unXm $ MkX m = m

v : Nat
v = unXm $ MkX 5

In this case, we get an error

Error: Unsolved holes:
 07 | unXm : X n -> Nat
 08 | unXm $ MkX m = m
 09 |
 10 | v : Nat
 11 | v = unXm $ MkX 5
          ^^^^^^^^^^^^

This error is since neither constructor MkX, nor function unXm cares about the value of parameter n, but compiler still needs to fill it some value, and cannot figure out any particular value himself.

If we really need

Sometimes we really need such expressions work, and we can name a value which compiler could choose for such a case. However, we don't want to lose an ability for such implicit values to be determined by type of some other context.

For this purpose, this library provides an interface IfUnsolved, which makes given implicit the default value only in the case, when this implicit would be unsolved otherwise:

data X : Nat -> Type where
  MkX : (0 _ : 42 `IfUnsolved` n) => (m : Nat) -> X n

In this case both previous examples would compile.

Here the value of the implicit is determined by type:

x2 : X 2
x2 = MkX 16

But in the case, when nothing explicitly determines the value, it is chosen by IfUnsolved and no error happens:

v : Nat
v = unXm $ MkX 5

Usage

The IfUnsolved interface resides in the Language.Implicits.IfUnsolved module.

If you use pack package manager, it is enough to add if-unsolved-implicit to depends section of your .ipkg-file, or to call pack install if-unsolved-implicit to install it directly.