Improve error reporting in coerce for unsupported coercions

[ablaom]

julia> X = (x=1:3,)
(x = 1:3,)

julia> coerce(X, :x=>Textual)
ERROR: MLJScientificTypes.CoercionError("`coerce` is undefined for non-tabular data.")

[ablaom]

There's something funny going on here. The error message implies the method coerce(::Val{:other}, X, a...; kw...) is dispatched (confirmed by stack trace) but this is impossible because trait(X) = :table and not :other as is immediately checked. I ran into this before and believe this mysterious wrong dispatch is related to JuliaAI/ScientificTypes.jl#104, and so ultimately to the julia bug JuliaLang/julia#36907 .

So let's revisit this after Julia 1.5.1 is released.

[OkonSamuel]

The behavior is same with julia 1.5.1 release

 julia> X = (x=1:3,)
(x = 1:3,)

julia> coerce(X, :x=>Textual)
ERROR: MLJScientificTypes.CoercionError("`coerce` is undefined for non-tabular data.")

julia> InteractiveUtils.versioninfo()
Julia Version 1.5.1
Commit 697e782ab8 (2020-08-25 20:08 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i3-5005U CPU @ 2.00GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-9.0.1 (ORCJIT, broadwell)

EDIT
But this time around the right method is dispatched as shown by the stack trace coerce(::Val{:table}, X, a...; kw...)

[OkonSamuel]

@ablaom. This issue is because there is currently no defined way of coercing 1:3 to Textual.
Defining a fall method should solve this issue. The fallback definition may be in the lines of.

function coerce(v, ::Type{T};
                verbosity::Int=1, tight::Bool=false
                )where T2 <: Union{Missing,Finite}
  throw(ArgumentError("Can't coerce $v which is of scitype $(scitype(v)) to scitype $."*
          "Or a better descriptive error message"))
 return
end

[ablaom]

What if we add

coerce(X::AbstractArray, a; kw...) =
    throw(CoercionError("Coercion to $a not supported for arrays of eltype $(eltype(X)). "))

?

This is ought to cover more cases, no?

[OkonSamuel]

This is ought to cover more cases, no?

no. The first case is more general and should cover all cases.
But We can still add more fallbacks like the example you gave above (which would give a specific error message when trying to coerce an array type). This would allow for better error messages at different levels.

[OkonSamuel]

On second thought the example you gave is actually more general since UnitRange <: AbstractArray.
I'll open a PR that closes this issue.