mirror of
https://github.com/idris-lang/Idris2.git
synced 2024-12-21 02:31:50 +03:00
48 lines
2.0 KiB
ReStructuredText
48 lines
2.0 KiB
ReStructuredText
Overloaded literals
|
||
===================
|
||
|
||
.. role:: idris(code)
|
||
:language: idris
|
||
|
||
The compiler provides directives for literals overloading, respectively
|
||
``%stringLit <fun>`` and ``%integerLit <fun>`` for string and integer literals. During
|
||
elaboration, the given function is applied to the corresponding literal.
|
||
In the Prelude these functions are set to ``fromString`` and ``fromInteger``.
|
||
|
||
The interface ``FromString ty`` provides the ``fromString : String -> ty`` function,
|
||
while the ``Num ty`` interface provides the ``fromInteger : Integer -> ty`` function
|
||
for all numerical types.
|
||
|
||
Restricted overloads
|
||
--------------------
|
||
Although the overloading of literals can be achieved by implementing the interfaces
|
||
described above, in principle only a function with the correct signature and name
|
||
is enough to achieve the desired behaviour. This can be exploited to obtain more
|
||
restrictive overloading such as converting literals to ``Fin n`` values, where
|
||
integer literals greater or equal to n are not constructible values for the type.
|
||
Additional implicit arguments can be added to the function signature, in particular
|
||
auto implicit arguments for searching proofs. As an example, this is the implementation
|
||
of ``fromInteger`` for ``Fin n``.
|
||
|
||
.. code-block:: idris
|
||
|
||
public export
|
||
fromInteger : (x : Integer) -> {n : Nat} ->
|
||
{auto prf : (IsJust (integerToFin x n))} ->
|
||
Fin n
|
||
fromInteger {n} x {prf} with (integerToFin x n)
|
||
fromInteger {n} x {prf = ItIsJust} | Just y = y
|
||
|
||
The ``prf`` auto implicit is an automatically constructed proof (if possible) that
|
||
the literal is suitable for the ``Fin n`` type. The restricted behaviour can be
|
||
observed in the REPL, where the failure to construct a valid proof is caught during
|
||
the type-checking phase and not at runtime:
|
||
|
||
.. code-block:: idris
|
||
|
||
Main> the (Fin 3) 2
|
||
FS (FS FZ)
|
||
Main> the (Fin 3) 5
|
||
(interactive):1:13--1:14:Can't find an implementation for IsJust (integerToFin 5 3) at:
|
||
1 the (Fin 3) 5
|