45 lines
1.8 KiB
Nim
45 lines
1.8 KiB
Nim
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import std/macros
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macro `<-`*(lhs: untyped, rhs: untyped): untyped =
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# ensure that the left-hand side is a valid bracket expression e.g. [a, b, c]
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let errmsg = "Invalid syntax for unpack operator: " & lhs.toStrLit().strVal()
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if lhs.kind != nnkBracket:
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error(errmsg, lhs)
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for name in lhs:
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if name.kind != nnkIdent:
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error(errmsg, lhs)
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# the result will be a check to ensure no index defects, followed by assignment statements
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result = newNimNode(nnkStmtList, lineInfoFrom = lhs)
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# first add a check to ensure that the container being unpacked has enough values
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# this has to be done at runtime, since it's potentially unknown at compile time
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let numIdents = lhs.len
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let strRepr = lhs.toStrLit().strVal() & " <- " & rhs.toStrLit().strVal()
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let lenCheck = quote do:
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if `rhs`.len < `numIdents`:
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raise newException(ValueError, "Not enough values to unpack: \"" & `strRepr` & "\"")
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result.add(lenCheck)
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var assign_from = rhs
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# if the right-hand side is an expression, it might
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# be expensive, so we save it to a temp variable
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if rhs.kind != nnkIdent:
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# repoint assign_src so that we end up using a temp variable
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# instead of repeating the original expression a bunch of times
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assign_from = ident("unpack_tmp_src")
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# e.g. "let unpack_tmp_src = somestring.split()"
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let declare_assign_src = newLetStmt(assign_from, rhs)
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result.add(declare_assign_src)
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# finally, inject the actual assignments
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for i, name in lhs:
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let assignment = quote do:
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# expanded to e.g. "let a = someseq[0]"
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let `name` = `assign_from`[`i`]
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result.add(assignment)
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proc showAddr*[T](x: T) = echo cast[uint64](unsafeAddr x)
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