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Add some more comments to record decoding
This commit is contained in:
Ayaz Hafiz
parent
b698e7f391
commit
9a7e280e9c
1 changed files with 47 additions and 18 deletions
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@ -40,8 +40,14 @@ pub(crate) fn derive_decoder(
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}
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}
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// theDecoder : Decoder {first: a, second: b} fmt | a has Decoding, b has Decoding, fmt has DecoderFormatting
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// theDecoder =
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// Implements decoding of a record. For example, for
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//
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// {first: a, second: b}
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//
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// we'd like to generate an impl like
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//
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// decoder : Decoder {first: a, second: b} fmt | a has Decoding, b has Decoding, fmt has DecoderFormatting
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// decoder =
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// initialState : {f0: Result a [NoField], f1: Result b [NoField]}
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// initialState = {f0: Err NoField, f1: Err NoField}
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//
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@ -69,18 +75,26 @@ pub(crate) fn derive_decoder(
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//
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// Decode.custom \bytes, fmt -> Decode.decodeWith bytes (Decode.record initialState stepField finalizer) fmt
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fn decoder_record(env: &mut Env, _def_symbol: Symbol, fields: Vec<Lowercase>) -> (Expr, Variable) {
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// The decoded type of each field in the record, e.g. {first: a, second: b}.
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let mut field_vars = Vec::with_capacity(fields.len());
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// The type of each field in the decoding state, e.g. {first: Result a [NoField], second: Result b [NoField]}
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let mut result_field_vars = Vec::with_capacity(fields.len());
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// initialState = ...
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let (initial_state_var, initial_state) =
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decoder_initial_state(env, &fields, &mut field_vars, &mut result_field_vars);
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let (finalizer, finalizer_var, decode_err_var) = decoder_finalizer(
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decoder_record_initial_state(env, &fields, &mut field_vars, &mut result_field_vars);
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// finalizer = ...
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let (finalizer, finalizer_var, decode_err_var) = decoder_record_finalizer(
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env,
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initial_state_var,
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&fields,
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&field_vars,
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&result_field_vars,
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);
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let (step_field, step_var) = decoder_step_field(
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// stepField = ...
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let (step_field, step_var) = decoder_record_step_field(
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env,
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fields,
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&field_vars,
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@ -89,6 +103,7 @@ fn decoder_record(env: &mut Env, _def_symbol: Symbol, fields: Vec<Lowercase>) ->
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decode_err_var,
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);
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// Build up the type of `Decode.record` we expect
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let record_decoder_var = env.subs.fresh_unnamed_flex_var();
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let decode_record_lambda_set = env.subs.fresh_unnamed_flex_var();
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let decode_record_var = env.import_builtin_symbol_var(Symbol::DECODE_RECORD);
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@ -157,16 +172,21 @@ fn decoder_record(env: &mut Env, _def_symbol: Symbol, fields: Vec<Lowercase>) ->
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// result: when rec.result is
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// Ok val -> Ok {state & first: Ok val},
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// Err err -> Err err
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// }
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// )
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// })
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//
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// "second" -> # We actually use the first branch's structure, which is a desugared version of this
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// "second" ->
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// Keep (Decode.custom \bytes, fmt ->
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// when Decode.decodeWith bytes Decode.decoder fmt is
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// {result, rest} ->
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// {result: Result.map result \val -> {state & second: Ok val}, rest})
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// rec ->
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// {
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// rest: rec.rest,
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// result: when rec.result is
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// Ok val -> Ok {state & second: Ok val},
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// Err err -> Err err
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// })
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//
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// _ -> Skip
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fn decoder_step_field(
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fn decoder_record_step_field(
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env: &mut Env,
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fields: Vec<Lowercase>,
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field_vars: &[Variable],
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@ -242,6 +262,7 @@ fn decoder_step_field(
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synth_var(env.subs, Content::Structure(tag_union))
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};
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// rec : { rest: List U8, result: (typeof rec.result) }
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let rec_var = {
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let fields = RecordFields::insert_into_subs(
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@ -256,6 +277,7 @@ fn decoder_step_field(
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synth_var(env.subs, Content::Structure(record))
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};
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// `Decode.decoder` for the field's value
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let decoder_var = env.import_builtin_symbol_var(Symbol::DECODE_DECODER);
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let decode_with_var = env.import_builtin_symbol_var(Symbol::DECODE_DECODE_WITH);
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let lambda_set_var = env.subs.fresh_unnamed_flex_var();
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@ -274,6 +296,7 @@ fn decoder_step_field(
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this_decode_with_var
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};
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// The result of decoding this field's value - either the updated state, or a decoding error.
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let when_expr_var = {
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let flat_type = FlatType::TagUnion(
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UnionTags::for_result(env.subs, state_record_var, decode_err_var),
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@ -282,6 +305,9 @@ fn decoder_step_field(
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synth_var(env.subs, Content::Structure(flat_type))
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};
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// What our decoder passed to `Decode.custom` returns - the result of decoding the
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// field's value, and the remaining bytes.
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custom_callback_ret_var = {
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let rest_field = RecordField::Required(Variable::LIST_U8);
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let result_field = RecordField::Required(when_expr_var);
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@ -587,8 +613,6 @@ fn decoder_step_field(
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let keep = {
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// Keep (Decode.custom \bytes, fmt ->
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// # Uses a single-branch `when` because `let` is more expensive to monomorphize
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// # due to checks for polymorphic expressions, and `rec` would be polymorphic.
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// when Decode.decodeWith bytes Decode.decoder fmt is
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// rec ->
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// {
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@ -609,8 +633,6 @@ fn decoder_step_field(
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let branch = {
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// "first" ->
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// Keep (Decode.custom \bytes, fmt ->
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// # Uses a single-branch `when` because `let` is more expensive to monomorphize
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// # due to checks for polymorphic expressions, and `rec` would be polymorphic.
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// when Decode.decodeWith bytes Decode.decoder fmt is
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// rec ->
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// {
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@ -714,11 +736,11 @@ fn decoder_step_field(
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// finalizer = \rec ->
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// when rec.first is
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// Ok first ->
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// when f1 is
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// when rec.second is
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// Ok second -> Ok {first, second}
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// Err NoField -> Err TooShort
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// Err NoField -> Err TooShort
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fn decoder_finalizer(
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fn decoder_record_finalizer(
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env: &mut Env,
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state_record_var: Variable,
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fields: &[Lowercase],
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@ -752,6 +774,8 @@ fn decoder_finalizer(
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fields_map.insert(field_name.clone(), field);
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}
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// The bottom of the happy path - return the decoded record {first: a, second: b} wrapped with
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// "Ok".
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let return_type_var;
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let mut body = {
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let subs = &mut env.subs;
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@ -786,6 +810,11 @@ fn decoder_finalizer(
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}
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};
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// Unwrap each result in the decoded state
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//
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// when rec.first is
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// Ok first -> ...happy path...
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// Err NoField -> Err TooShort
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for (((symbol, field_name), &field_var), &result_field_var) in pattern_symbols
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.iter()
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.rev()
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@ -893,7 +922,7 @@ fn decoder_finalizer(
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// Example:
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// initialState : {first: Result a [NoField], second: Result b [NoField]}
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// initialState = {first: Err NoField, second: Err NoField}
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fn decoder_initial_state(
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fn decoder_record_initial_state(
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env: &mut Env<'_>,
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field_names: &[Lowercase],
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field_vars: &mut Vec<Variable>,
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