43 lines
1.2 KiB
Nim
43 lines
1.2 KiB
Nim
# stew
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# Copyright 2024 Status Research & Development GmbH
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# Licensed under either of
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#
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# * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
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# * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
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#
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# at your option. This file may not be copied, modified, or distributed except according to those terms.
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import std/macros
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macro evalOnceAs*(exp, alias: untyped): untyped =
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## Ensure that `exp` is evaluated only once unless it is a symbol in which
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## case it's used directly.
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##
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## A common case where this is useful is template parameters which, when
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## an expression is passed in, get evaluated multiple times.
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##
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## Based on a similar macro in std/sequtils
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expectKind(alias, nnkIdent)
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let
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body = nnkStmtList.newTree()
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val =
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if exp.kind == nnkSym:
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# The symbol can be used directly
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# TODO dot expressions? etc..
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exp
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else:
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let val = genSym(ident = "evalOnce_" & $alias)
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body.add newLetStmt(val, exp)
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val
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body.add(
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newProc(
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name = genSym(nskTemplate, $alias),
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params = [getType(untyped)],
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body = val,
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procType = nnkTemplateDef,
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)
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)
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body
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