EltypeExtensions

EltypeExtensions.jl is a mini toolbox for eltype-related conversions. The motivation of this package comes from manipulating (nested) arrays with different eltypes. However if you have any reasonable idea that works on other instances, feel free to write an issue/pull request.

We note that this package has some overlap with TypeUtils.jl and Unitless.jl.

elconvert(T, x) works like convert(T, x), except that T refers to the eltype of the result. This can be useful for generic codes.

It should be always true that elconvert(T, x) isa _to_eltype(T, typeof(x)). However, since elconvert and _to_eltype use different routines, it's possible that the equality doesn't hold for some types. Please submit an issue or PR if that happens.

If typeof(x) is not in Base or stdlib, the package who owns the type should implement corresponding _to_eltype or elconvert. elconvert has fallbacks, in which case it could be unnecessary:

• For a subtype of AbstractArray, elconvert calls the constructor AbstractArray{T} and _to_eltype returns Array.
• For a subtype of AbstractUnitRange, elconvert calls the constructor AbstractUnitRange{T}.
• For a subtype of AbstractRange, elconvert uses broadcast through map.
• For a Tuple, elconvert uses dot broadcast.
• For other types, elconvert calls convert and _to_eltype.

However, _to_eltype must be implemented for each type to support baseconvert and precisionconvert. The following types from Base and stdlib are explicitly supported by _to_eltype:

AbstractArray, AbstractDict, AbstractSet, Adjoint, Bidiagonal, BitArray,
CartesianIndices, Diagonal, Dict, Hermitian, Set, StepRangeLen, Symmetric,
SymTridiagonal, Transpose, TwicePrecision, UnitRange

The basetype is used for nested collections, where eltype is repeatedly applied until the bottom. precisiontype has a similar idea, but goes deeper when possible. precisiontype is used to manipulate the accuracy of (nested) collections.

• sometype(T) gets the sometype of type T.
• sometype(x) = sometype(typeof(x)) is also provided for convenience.
• _to_sometype(T,S) converts the type S to have the sometype of T.
• someconvert(T,A) converts A to have the sometype of T.

where some can be el, base and precision.

precisionconvert accepts an optional third argument prec.

• When T has static precision, prec has no effect.
• When T has dynamic precision, prec specifies the precision of conversion. When prec is not provided, the precision is decided by the external setup from T. The difference is significant when precisionconvert is called by another function. See the document for an example.
• When T is an integer, the conversion will dig into Rational as well. In contrast, since Rational as a whole is more "precise" than an integer, precisiontype doesn't unwrap Rational.