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January 14, 2020 19:24
`vcomp` macro: vector comprehensions with conditional `when` clause for Julia.
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| """ | |
| @vcomp(comprehension::Expr, when::Symbol, condition) | |
| `@vcomp` (*vector comprehension*) based on Python's list comprehensions | |
| with a conditional clause to filter elements in a succinct way. | |
| # Examples | |
| ```julia | |
| julia> @vcomp Int[i^2 for i in 1:10] when i % 2 == 0 | |
| 5-element Array{Int,1}: | |
| 4 | |
| 16 | |
| 36 | |
| 64 | |
| 100 | |
| julia> words = ["ant", "akita", "bear", "cow"]; | |
| julia> @vcomp [uppercase(word) for word in words] when startswith(word, "a") | |
| 2-element Array{Any,1}: | |
| "ANT" | |
| "AKITA" | |
| julia> @vcomp Tuple[(i, e) for (i, e) in enumerate(reverse(1:10))] when i-e < 0 | |
| 5-element Array{Tuple,1}: | |
| (1,10) | |
| (2,9) | |
| (3,8) | |
| (4,7) | |
| (5,6) | |
| ``` | |
| """ | |
| macro vcomp(comprehension::Expr, when::Symbol, condition) | |
| comp_head, comp_args = comprehension.head, comprehension.args | |
| comp_head ∉ [:comprehension, :typed_comprehension] && error("@vcomp not a comprehension") | |
| when ≠ :when && error("@vcomp expected `when`, got: `$when`") | |
| T = comp_head == :typed_comprehension ? comp_args[1] : nothing | |
| if VERSION < v"0.5-" | |
| element = comp_head == :comprehension ? comp_args[1] : comp_args[2] | |
| sequence = comp_head == :comprehension ? comp_args[2] : comp_args[3] | |
| else | |
| element = comp_head == :comprehension ? comp_args[1].args[1] : comp_args[2].args[1] | |
| sequence = comp_head == :comprehension ? comp_args[1].args[2] : comp_args[2].args[2] | |
| end | |
| result = T ≠ nothing ? :($T[]) : :([]) | |
| block = Expr(:let, Expr(:block, | |
| Expr(:(=), :res, result), | |
| Expr(:for, sequence, | |
| Expr(:if, condition, | |
| Expr(:call, :push!, :res, element))), | |
| :res)) | |
| return esc(block) | |
| end |
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