Inconsistent tuple pattern matching

[evanfields]

Julia allows pattern matching on tuples in some cases (iteration) but not others (applying anonymous functions). This isn't a major issue, but it does seem a bit of a design inconsistency and could be confusing to new users:

julia> versioninfo()
Julia Version 0.5.0
Commit 3c9d753 (2016-09-19 18:14 UTC)
Platform Info:
  System: NT (x86_64-w64-mingw32)
  CPU: Intel(R) Core(TM) i5-2400 CPU @ 3.10GHz
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Sandybridge)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.7.1 (ORCJIT, sandybridge)

julia> x = [(1,2),(3,4),(5,6)]
3-element Array{Tuple{Int64,Int64},1}:
 (1,2)
 (3,4)
 (5,6)

julia> for (a,b) in x
           println(b)
       end
2
4
6

julia> [b for (a,b) in x]
3-element Array{Int64,1}:
 2
 4
 6

julia> sum(b for (a,b) in x)
12

julia> map((a,b) -> b, x)
ERROR: MethodError: no method matching (::##125#126)(::Tuple{Int64,Int64})
Closest candidates are:
  #125(::Any, ::Any) at REPL[20]:1
 in _collect(::Array{Tuple{Int64,Int64},1}, ::Base.Generator{Array{Tuple{Int64,Int64},1},##125#126}, ::Base.EltypeUnknown, ::Base.HasShape) at .\array.jl:320
 in map(::Function, ::Array{Tuple{Int64,Int64},1}) at .\abstractarray.jl:1683

[yuyichao]

What's the inconsistency? The function you want to map to takes the same number of arguments as the number of collections you are mapping to.

[JeffBezanson]

See #6614. The syntax in that case would be map( ((a,b),)->b, x ).

It's vaguely possible to have a language where a function of 1 2-tuple is the same as a function of 2 arguments, but overall I think that leads to worse problems and more confusion.

[KristofferC]

Seems like there is nothing actionable here.