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Add conditional expressions #536

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13 changes: 13 additions & 0 deletions askama_derive/src/generator.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -756,6 +756,19 @@ enum DisplayWrap {
Unwrapped,
}

/// `Wrapped & Wrapped == Wrapped`, otherwise `Unwrapped`
impl std::ops::BitAnd for DisplayWrap {
type Output = Self;

#[inline]
fn bitand(self, rhs: Self) -> Self::Output {
match self {
DisplayWrap::Wrapped => rhs,
DisplayWrap::Unwrapped => self,
}
}
}

#[derive(Default, Debug)]
struct WritableBuffer<'a> {
buf: Vec<Writable<'a>>,
Expand Down
34 changes: 34 additions & 0 deletions askama_derive/src/generator/expr.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
use std::borrow::Cow;
use std::str::FromStr;

use parser::expr::Conditional;
use parser::node::CondTest;
use parser::{
AssociatedItem, CharLit, CharPrefix, Expr, PathComponent, Span, StrLit, StrPrefix, Target,
Expand All @@ -14,6 +15,7 @@ use super::{
DisplayWrap, Generator, LocalMeta, Writable, binary_op, compile_time_escape, is_copyable,
logic_op, range_op, unary_op,
};
use crate::generator::node::EvaluatedResult;
use crate::heritage::Context;
use crate::integration::Buffer;
use crate::{CompileError, field_new, quote_into};
Expand Down Expand Up @@ -103,6 +105,7 @@ impl<'a> Generator<'a, '_> {
Expr::Concat(ref exprs) => self.visit_concat(ctx, buf, exprs)?,
Expr::LetCond(ref cond) => self.visit_let_cond(ctx, buf, cond)?,
Expr::ArgumentPlaceholder => DisplayWrap::Unwrapped,
Expr::Conditional(ref cond) => self.visit_conditional_expr(ctx, buf, cond)?,
})
}

Expand Down Expand Up @@ -251,6 +254,37 @@ impl<'a> Generator<'a, '_> {
}
}

fn visit_conditional_expr(
&mut self,
ctx: &Context<'_>,
buf: &mut Buffer,
cond: &Conditional<'a>,
) -> Result<DisplayWrap, CompileError> {
let result;
let expr = if cond.test.contains_bool_lit_or_is_defined() {
result = self.evaluate_condition(WithSpan::clone(&cond.test), &mut true);
match &result {
EvaluatedResult::AlwaysTrue => return self.visit_expr(ctx, buf, &cond.then),
EvaluatedResult::AlwaysFalse => return self.visit_expr(ctx, buf, &cond.otherwise),
EvaluatedResult::Unknown(expr) => expr,
}
} else {
&cond.test
};

let mut then_buf = Buffer::new();
let then = self.visit_expr(ctx, &mut then_buf, &cond.then)?;
let mut otherwise_buf = Buffer::new();
let otherwise = self.visit_expr(ctx, &mut otherwise_buf, &cond.otherwise)?;

let span = ctx.span_for_node(expr.span());
buf.write_token(Token![if], span);
self.visit_condition(ctx, buf, expr)?;
quote_into!(buf, span, { { #then_buf } else { #otherwise_buf } });

Ok(then & otherwise)
}

fn visit_let_cond(
&mut self,
ctx: &Context<'_>,
Expand Down
16 changes: 14 additions & 2 deletions askama_derive/src/generator/node.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -187,7 +187,7 @@ impl<'a> Generator<'a, '_> {
Ok(size_hint)
}

fn evaluate_condition(
pub(super) fn evaluate_condition(
&self,
expr: WithSpan<'a, Box<Expr<'a>>>,
only_contains_is_defined: &mut bool,
Expand Down Expand Up @@ -310,6 +310,15 @@ impl<'a> Generator<'a, '_> {
EvaluatedResult::AlwaysTrue
}
}
Expr::Conditional(cond) => {
// If we can tell the outcome of the condition, recurse.
let expr = match self.evaluate_condition(cond.test, only_contains_is_defined) {
EvaluatedResult::AlwaysTrue => cond.then,
EvaluatedResult::AlwaysFalse => cond.otherwise,
EvaluatedResult::Unknown(expr) => return EvaluatedResult::Unknown(expr),
};
self.evaluate_condition(expr, only_contains_is_defined)
}
}
}

Expand Down Expand Up @@ -1500,7 +1509,7 @@ struct Conds<'a> {
}

#[derive(Debug, Clone, PartialEq)]
enum EvaluatedResult<'a> {
pub(super) enum EvaluatedResult<'a> {
AlwaysTrue,
AlwaysFalse,
Unknown(WithSpan<'a, Box<Expr<'a>>>),
Expand Down Expand Up @@ -1660,6 +1669,9 @@ fn is_cacheable(expr: &WithSpan<'_, Box<Expr<'_>>>) -> bool {
Expr::As(expr, _) => is_cacheable(expr),
Expr::Try(expr) => is_cacheable(expr),
Expr::Concat(args) => args.iter().all(is_cacheable),
Expr::Conditional(cond) => {
is_cacheable(&cond.test) && is_cacheable(&cond.then) && is_cacheable(&cond.otherwise)
}
// Doesn't make sense in this context.
Expr::LetCond(_) => false,
// We have too little information to tell if the expression is pure:
Expand Down
127 changes: 124 additions & 3 deletions askama_derive/src/tests.rs
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Original file line number Diff line number Diff line change
@@ -1,7 +1,9 @@
//! Files containing tests for generated code.

use std::fmt;
use std::io::Write;
use std::path::Path;
use std::process::exit;
use std::{fmt, io};

use console::style;
use prettyplease::unparse;
Expand Down Expand Up @@ -124,7 +126,8 @@ fn compare_ex(
}
}

panic!(
let _: io::Result<()> = writeln!(
io::stderr().lock(),
"\n\
=== Expected ===\n\
\n\
Expand All @@ -138,11 +141,15 @@ fn compare_ex(
\n\
{diff}\n\
\n\
=== FAILURE ===",
=== FAILURE ===\n\
\n\
{location}\n",
expected = style(&expected).red(),
generated = style(&generated).green(),
diff = Diff(&expected, &generated),
location = std::panic::Location::caller(),
);
exit(1);
}
}

Expand Down Expand Up @@ -1443,3 +1450,117 @@ fn check_expr_ungrouping() {
11,
);
}

#[test]
fn test_conditional_expr() {
compare(
r"{{ a if b else c }}",
r#"
match (
&((&&askama::filters::AutoEscaper::new(
&(if askama::helpers::as_bool(&(self.b)) { self.a } else { self.c }),
askama::filters::Text,
))
.askama_auto_escape()?),
) {
(__askama_expr0,) => {
(&&&askama::filters::Writable(__askama_expr0))
.askama_write(__askama_writer, __askama_values)?;
}
}"#,
&[("a", "i8"), ("b", "i8"), ("c", "i8")],
3,
);

compare(
r"{{ a if b is defined else c }}",
r#"
match (
&((&&askama::filters::AutoEscaper::new(&(self.a), askama::filters::Text))
.askama_auto_escape()?),
) {
(__askama_expr0,) => {
(&&&askama::filters::Writable(__askama_expr0))
.askama_write(__askama_writer, __askama_values)?;
}
}"#,
&[("a", "i8"), ("b", "i8"), ("c", "i8")],
3,
);

compare(
r"{{ a if b is not defined else c }}",
r#"
match (
&((&&askama::filters::AutoEscaper::new(&(self.c), askama::filters::Text))
.askama_auto_escape()?),
) {
(__askama_expr0,) => {
(&&&askama::filters::Writable(__askama_expr0))
.askama_write(__askama_writer, __askama_values)?;
}
}"#,
&[("a", "i8"), ("b", "i8"), ("c", "i8")],
3,
);

compare(
r"{{ a if b if c else d else e }}",
r#"
match (
&((&&askama::filters::AutoEscaper::new(
&(if askama::helpers::as_bool(
&(if askama::helpers::as_bool(&(self.c)) { self.b } else { self.d })
) {
self.a
} else {
self.e
}),
askama::filters::Text,
))
.askama_auto_escape()?),
) {
(__askama_expr0,) => {
(&&&askama::filters::Writable(__askama_expr0))
.askama_write(__askama_writer, __askama_values)?;
}
}"#,
&[
("a", "i8"),
("b", "i8"),
("c", "i8"),
("d", "i8"),
("e", "i8"),
],
3,
);

compare(
r"{{ a if b else c if d else e }}",
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Collaborator

@GuillaumeGomez GuillaumeGomez Jul 23, 2025 •
edited
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This is absolutely horrifying for my brain to parse. 🤣

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What about this formatting? :)

{{
   a if b else
   c if d else
   e
}}

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That doesn't help much, I'm too used to blocks. But it's fine, don't worry. One question though, does this work:

{{ a if b else (c if d else e) }}

?

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Yes, that does work.

r#"
match (
&((&&askama::filters::AutoEscaper::new(
&(if askama::helpers::as_bool(&(self.b)) {
self.a
} else {
if askama::helpers::as_bool(&(self.d)) { self.c } else { self.e }
}),
askama::filters::Text,
))
.askama_auto_escape()?),
) {
(__askama_expr0,) => {
(&&&askama::filters::Writable(__askama_expr0))
.askama_write(__askama_writer, __askama_values)?;
}
}"#,
&[
("a", "i8"),
("b", "i8"),
("c", "i8"),
("d", "i8"),
("e", "i8"),
],
3,
);
}
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