RFC: Initialize runtime globals during __init__ #703
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,124 @@ | ||
| import Base.Meta: isexpr | ||
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| """ | ||
| ```julia | ||
| @defconstants H5PREFIX begin | ||
| STATIC_CONSTANT::hid_t = 1 | ||
| DYNAMIC_CONSTANT::hid_t | ||
| ... | ||
| end | ||
| ``` | ||
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| Defines a collection of "constants" (`STATIC_CONSTANT`, `DYANMIC_CONSTANT`, ...) within | ||
| the namespace `H5PREFIX` wherein the distinction between runtime and static values is | ||
| abstracted away from the user's view. | ||
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| For truly constant values such as `STATIC_CONSTANT`, the syntax is given as in the first | ||
| line above where the value is assigned to a name with a type assertion. The value may also | ||
| be an expression which is evaluated the current module context. Note that the value | ||
| evaluation occurs before the constants are assigned, so the expression cannot refer to | ||
| any previous constants within the same block; e.g. the following will not work: | ||
| ```julia | ||
| julia> @defconstants BITMASK begin | ||
| BIT_ONE::UInt = 0x1 | ||
| BIT_TWO::UInt = 0x2 | ||
| BIT_BOTH::Uint = BIT_ONE | BIT_TWO | ||
| end | ||
| ERROR: LoadError: UndefVarError: BIT_ONE not defined | ||
| ... | ||
| ``` | ||
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| The second permitted syntax is for runtime constants which require initialization. | ||
| In this form, only the name and value type are given, and an appropriately-typed, | ||
| uninitialized `Ref` container is assigned to the constant binding. These values must be | ||
| assigned at runtime from an `__init__()` function: | ||
| ```julia | ||
| function __init__() | ||
| H5PREFIX.DYANMIC_CONSTANT = my_runtime_value() | ||
| end | ||
| ``` | ||
| """ | ||
| macro defconstants(prefix::Symbol, expr::Expr) | ||
| isexpr(expr, :block) || error("Expected block expression") | ||
| stmts = expr.args | ||
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| symbols = Vector{Symbol}(undef, 0) | ||
| imports = Vector{Union{Symbol,Expr}}(undef, 0) | ||
| defbody = Vector{Expr}(undef, 0) | ||
| getbody = Vector{Expr}(undef, 0) | ||
| setbody = Vector{Expr}(undef, 0) | ||
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| innermod = Symbol(:_, prefix) | ||
| for line in stmts | ||
| line isa LineNumberNode && continue | ||
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| isruntime = isexpr(line, :(=)) ? false : | ||
| isexpr(line, :(::)) ? true : | ||
| error("Unexpected statement: ", line) | ||
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| # Get the name and type pair | ||
| nametype = isruntime ? line : line.args[1] | ||
| isexpr(nametype, :(::)) || error("Expected `name::type`, got: ", nametype) | ||
| name = nametype.args[1]::Symbol | ||
| type = nametype.args[2]::Union{Symbol,Expr} | ||
| # Save type for later use | ||
| push!(imports, type) | ||
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| if isruntime | ||
| value = esc(:(Ref{$type}())) | ||
| else | ||
| valexpr = :(convert($type, $(line.args[2]))) | ||
| value = QuoteNode(Core.eval(__module__, valexpr)) | ||
| end | ||
| fullname = esc(:($innermod.$name)) | ||
| getexpr = isruntime ? :($(fullname)[]) : fullname | ||
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| push!(symbols, name) | ||
| push!(defbody, :(const $(esc(name)) = $value)) | ||
| push!(getbody, :(sym === $(QuoteNode(name)) && return $getexpr)) | ||
| if isruntime | ||
| setexpr = quote | ||
| sym === $(QuoteNode(name)) && begin | ||
| $fullname[] = value | ||
| return value | ||
| end | ||
| end | ||
| append!(setbody, Base.remove_linenums!(setexpr).args) | ||
| end | ||
| end | ||
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| # Build expressions to import all necessary types from the parent module. | ||
| imports = Expr[Expr(:(.), :(.), :(.), sym) for sym in unique!(imports)] | ||
| # Push the imports into the definition body | ||
| pushfirst!(defbody, Expr(:import, imports...)) | ||
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| eprefix = esc(prefix) | ||
| einnermod = esc(innermod) | ||
| block = quote | ||
| baremodule $einnermod | ||
| struct $eprefix end | ||
| $(defbody...) | ||
| end | ||
| const $eprefix = $einnermod.$prefix() | ||
| function Base.propertynames(::$einnermod.$prefix) | ||
| return $((symbols...,)) | ||
| end | ||
| function Base.getproperty(::$einnermod.$prefix, sym::Symbol) | ||
| $(getbody...) | ||
| error($(string(prefix) * " has no constant "), sym) | ||
| end | ||
| end | ||
| if !isempty(setbody) | ||
| setfn = quote | ||
| function Base.setproperty!(::$einnermod.$prefix, sym::Symbol, value) | ||
| $(setbody...) | ||
| error($(string(prefix) * "."), sym, " cannot be set") | ||
|
There was a problem hiding this comment. We should probably change the error message to be consistent with what is printed in Base. Either: Assigning a variable to in an other module: Trying to redefine a constant: There was a problem hiding this comment. I like the second one because it's still technically correct. |
||
| end | ||
| end | ||
| append!(block.args, Base.remove_linenums!(setfn).args) | ||
| end | ||
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| Base.remove_linenums!(block) | ||
| block.head = :toplevel | ||
| return block | ||
| end | ||
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Maybe better to
Base.remove_linenums!(expr1)first ?There was a problem hiding this comment.
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Possibly — I don't know which is faster to execute. I was guessing the
isa-continuemight be since it wouldn't have to modify any arrays.