Having trouble finding the right sink / callback combo for a function call postfix operator #237
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by
foonathan
natinusala
asked this question in
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Hello, I am trying to make a parser for a toy programming language. I am trying to implement function calls as a postfix operator in a Lexy expression. I took example from the calculator example. This is the expression production: struct nested_expression: lexy::transparent_production {
static constexpr auto rule = dsl::recurse<struct expression>;
static constexpr auto value = lexy::forward<ExpressionSyntaxPtr>;
};
struct expression: lexy::expression_production {
static constexpr auto whitespace = dsl::ascii::blank;
static constexpr auto atom = dsl::parenthesized(dsl::p<nested_expression>) | dsl::p<integer_literal> | dsl::p<identifier>;
struct addition: dsl::infix_op_left {
static constexpr auto op = dsl::op<BinaryOperatorExpressionSyntax::PLUS>(LEXY_LIT("+")) / dsl::op<BinaryOperatorExpressionSyntax::MINUS>(LEXY_LIT("-"));
using operand = dsl::atom;
};
struct function_call: dsl::postfix_op {
static constexpr auto op = dsl::op<void>(dsl::parenthesized.opt_list(dsl::p<nested_expression>, dsl::trailing_sep(dsl::comma)));
using operand = addition;
};
using operation = function_call;
static constexpr auto value = lexy::callback<ExpressionSyntaxPtr>(
// Atoms
lexy::forward<ExpressionSyntaxPtr>,
lexy::new_<IntegerLiteralExpressionSyntax, ExpressionSyntaxPtr>,
lexy::new_<IdentifierExpressionSyntax, ExpressionSyntaxPtr>,
// Operators
lexy::new_<BinaryOperatorExpressionSyntax, ExpressionSyntaxPtr>,
// Function call
lexy::new_<FunctionCallExpressionSyntax, ExpressionSyntaxPtr>
);
};
using ExpressionSyntaxPtr = std::shared_ptr<struct ExpressionSyntax>;
struct ExpressionSyntax { /* ... */ };
struct FunctionCallExpressionSyntax: public ExpressionSyntax {
ExpressionSyntaxPtr called_expression;
std::vector<ExpressionSyntaxPtr> arguments;
explicit FunctionCallExpressionSyntax(ExpressionSyntaxPtr called_expression, std::vector<ExpressionSyntaxPtr> arguments): called_expression(called_expression), arguments(arguments) {}
// ...
}When trying to compile this, I have the error "missing value sink for production". I understand that this comes from the From what I understand from the documentation, I need to use a sink with the I think there is a right combination of sink / callback to use but I have a hard time figuring what it is. |
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Answered by
foonathan
Jun 8, 2025
Replies: 1 comment 2 replies
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I think the issue is that FYI, the static assert "missing value callback for production" shows you the callback that is missing in the error message in the types of the This means "the callback for |
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2 replies
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Using As for the error message, sadly this seems to be a clang-only feature. I'm on MSVC (VS2022) and this is the error I have: Error collapsedFirst of all it's in french so it's horrible to find solutions to some problems, but more importantly there are no mentions of |
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Argh, that's unfortunate. |
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Answer selected by
natinusala
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I think the issue is that
opt_listproduceslexy::nulloptwhen the list is not empty, not an empty sink: https://lexy.foonathan.net/reference/dsl/terminator/#_rule_opt_list So you need to add a constructor toFunctionCallExpressionSyntaxthat acceptsstd::nullopt, or change it to one that constructs astd::optional<std::vector<...>>.FYI, the static assert "missing value callback for production" shows you the callback that is missing in the error message in the types of the
_detail::errorinstantiation. For example: https://godbolt.org/z/6ojrMYWf5 fails with