no method round(MathConst{:π})

[ihnorton]

julia> round(pi)
ERROR: no method round(MathConst{:π})

reported here

[ihnorton]

The Forio tutorial could be updated to use the 2-arg form. But there is also inconsistent behavior like this:

julia> round(-pi)
-3.0

[JeffBezanson]

Other than conversion-related functions, only +,-,*,/,^ are implemented for MathConsts. Adding every function is not practical.

So, we obviously need a more systematic approach for "exotic" kinds of numbers that are not going to have every function implemented just for them. MathConst and Float16 are good examples --- rather than re-implement all of libm for them, we want to convert to a standard type, then possibly convert back. Perhaps a function like stdfloat(x) that converts x to an appropriate standard type (Float32, Float64, or BigFloat --- those should be sufficient).

[nolta]

What about just bringing back the old definition of pi, but keeping the MathConst π? Then we get the best of both worlds.

[StefanKarpinski]

This could be partially handled with generic apply. I.e. apply(f::Function, x::MathConst) = f(float(x)).

[StefanKarpinski]

@nolta: forcing people to use Unicode to get a correctly behaved generic pi seems wrong.

[nolta]

Call it Pi then, just like Mathematica.

[JeffBezanson]

A definition like that would be ambiguous with an ordinary definition of a particular generic function for a type like Real.

[quinnj]

We could just prompt the user to do their own conversion, though that still requires a round(::MathConst) definition of sorts, unless the apply thing could be made to work (i.e. trying to apply any function to a MathConst would simple error with, please convert me to Float64, BigFloat, etc)

[NHDaly]

This issue is marked as up for grabs and intro issue, but in light of the conversation at #21535 (Irrational arithmetic is a slippery, dangerous slope), this seems like more of a decision?

[tpapp]

Would the missing method

julia> round(Int, pi)
ERROR: MethodError: no method matching round(::Type{Int64}, ::Irrational{:π})
Closest candidates are:
  round(::Type{T<:Integer}, ::Integer) where T<:Integer at int.jl:524
  round(::Type{T<:Integer}, ::Float16) where T<:Integer at float.jl:356
  round(::Type{T<:Union{Signed, Unsigned}}, ::BigFloat) where T<:Union{Signed, Unsigned} at mpfr.jl:229
  ...

julia> VERSION
v"0.7.0-DEV.3507"

fall under this issue? Asking because so much of the interface changed since it was opened.

round(pi, [digits]) works.

[KristofferC]

round(pi) now works. For adding additional methods, closing based on #5561 (comment).