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@freedomtowin
Created June 28, 2025 21:17
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Example of creating a Gist using Python
impl ToTokens for Comprehension {
fn to_tokens(&self, tokens: &mut TokenStream2) {
let all_for_if_clauses =
std::iter::once(&self.for_if_clause).chain(&self.additional_for_if_clauses);
let mut innermost_to_outermost = all_for_if_clauses.rev();
let mut output = {
// innermost is a special case--here we do the mapping
let innermost = innermost_to_outermost
.next()
.expect("We know we have at least one ForIfClause (self.for_if_clause)");
let ForIfClause {
pattern,
sequence,
conditions,
} = innermost;
let Mapping(mapping) = &self.mapping;
quote! {
::core::iter::IntoIterator::into_iter(#sequence).filter_map(move |#pattern| {
(true #(&& (#conditions))*).then(|| #mapping)
})
}
};
// Now we walk through the rest of the ForIfClauses, wrapping the current `output` in a new layer of iteration each time.
// We also add an extra call to '.flatten()'.
output = innermost_to_outermost.fold(output, |current_output, next_layer| {
let ForIfClause {
pattern,
sequence,
conditions,
} = next_layer;
quote! {
::core::iter::IntoIterator::into_iter(#sequence).filter_map(move |#pattern| {
(true #(&& (#conditions))*).then(|| #current_output)
})
.flatten()
}
});
tokens.extend(output)
}
}
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