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expand record pattern matches to bind all unused fields with underscore

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this will simplify exhaustiveness checking, because a record is not like a one-tag union
This commit is contained in:
Folkert 2020-03-19 20:39:14 +01:00
parent 5fc96c09ee
commit 0985037754
1 changed files with 49 additions and 9 deletions
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50
compiler/mono/src/expr.rs
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@ -942,6 +942,19 @@ fn store_record_destruct<'a>(
Pattern::Identifier(symbol) => {
stored.push((*symbol, destruct.layout.clone(), load));
}
Pattern::Underscore => {
// important that this is special-cased: mono record patterns will extract all the
// fields, but those not bound in the source code are guarded with the underscore
// pattern. So given some record `{ x : a, y : b }`, a match
//
// { x } -> ...
//
// is actually
//
// { x, y: _ } -> ...
//
// internally. But `y` is never used, so we must make sure it't not stored/loaded.
}
_ => {
let symbol = env.fresh_symbol();
stored.push((symbol, destruct.layout.clone(), load));
@ -1320,15 +1333,42 @@ fn from_can_pattern<'a>(
} => match Layout::from_var(env.arena, *whole_var, env.subs, env.pointer_size) {
Ok(Layout::Struct(field_layouts)) => {
let mut mono_destructs = Vec::with_capacity_in(destructs.len(), env.arena);
for (loc_rec_des, (label, field_layout)) in
destructs.iter().zip(field_layouts.iter())
{
debug_assert!(&loc_rec_des.value.label == label);
let mut destructs = destructs.clone();
destructs.sort_by(|a, b| a.value.label.cmp(&b.value.label));
let mut it = destructs.iter();
let mut opt_destruct = it.next();
// insert underscore patterns for unused fields. We need the record to be fully
// matched for pattern exhaustiveness checking
for (label, field_layout) in field_layouts.iter() {
if let Some(destruct) = opt_destruct {
if &destruct.value.label == label {
opt_destruct = it.next();
mono_destructs.push(from_can_record_destruct(
env,
&loc_rec_des.value,
&destruct.value,
field_layout.clone(),
));
} else {
// insert underscore pattern
mono_destructs.push(RecordDestruct {
label: label.clone(),
symbol: env.fresh_symbol(),
layout: field_layout.clone(),
guard: Some(Pattern::Underscore),
});
}
} else {
// insert underscore pattern
mono_destructs.push(RecordDestruct {
label: label.clone(),
symbol: env.fresh_symbol(),
layout: field_layout.clone(),
guard: Some(Pattern::Underscore),
});
}
}
Pattern::RecordDestructure(mono_destructs, Layout::Struct(field_layouts))