Factor out decoders into separate files

crates/compiler/derive/src/decoding/list.rs

@@ -0,0 +1,104 @@
+use roc_can::expr::Expr;
+
+use roc_error_macros::internal_error;
+use roc_module::called_via::CalledVia;
+
+use roc_module::symbol::Symbol;
+use roc_region::all::Loc;
+use roc_types::subs::{Content, FlatType, GetSubsSlice, SubsSlice, Variable};
+use roc_types::types::AliasKind;
+
+use crate::decoding::wrap_in_decode_custom_decode_with;
+use crate::synth_var;
+use crate::util::Env;
+
+pub fn decoder(env: &mut Env<'_>, _def_symbol: Symbol) -> (Expr, Variable) {
+    // Build
+    //
+    //   def_symbol : Decoder (List elem) fmt | elem has Decoding, fmt has DecoderFormatting
+    //   def_symbol = Decode.custom \bytes, fmt -> Decode.decodeWith bytes (Decode.list Decode.decoder) fmt
+    //
+    // NB: reduction to `Decode.list Decode.decoder` is not possible to the HRR.
+
+    use Expr::*;
+
+    // Decode.list Decode.decoder : Decoder (List elem) fmt
+    let (decode_list_call, this_decode_list_ret_var) = {
+        // List elem
+        let elem_var = env.subs.fresh_unnamed_flex_var();
+
+        // Decode.decoder : Decoder elem fmt | elem has Decoding, fmt has EncoderFormatting
+        let (elem_decoder, elem_decoder_var) = {
+            // build `Decode.decoder : Decoder elem fmt` type
+            // Decoder val fmt | val has Decoding, fmt has EncoderFormatting
+            let elem_decoder_var = env.import_builtin_symbol_var(Symbol::DECODE_DECODER);
+
+            // set val ~ elem
+            let val_var = match env.subs.get_content_without_compacting(elem_decoder_var) {
+                Content::Alias(Symbol::DECODE_DECODER_OPAQUE, vars, _, AliasKind::Opaque)
+                    if vars.type_variables_len == 2 =>
+                {
+                    env.subs.get_subs_slice(vars.type_variables())[0]
+                }
+                _ => internal_error!("Decode.decode not an opaque type"),
+            };
+
+            env.unify(val_var, elem_var);
+
+            (
+                AbilityMember(Symbol::DECODE_DECODER, None, elem_decoder_var),
+                elem_decoder_var,
+            )
+        };
+
+        // Build `Decode.list Decode.decoder` type
+        // Decoder val fmt -[uls]-> Decoder (List val) fmt | fmt has DecoderFormatting
+        let decode_list_fn_var = env.import_builtin_symbol_var(Symbol::DECODE_LIST);
+
+        // Decoder elem fmt -a-> b
+        let elem_decoder_var_slice = SubsSlice::insert_into_subs(env.subs, [elem_decoder_var]);
+        let this_decode_list_clos_var = env.subs.fresh_unnamed_flex_var();
+        let this_decode_list_ret_var = env.subs.fresh_unnamed_flex_var();
+        let this_decode_list_fn_var = synth_var(
+            env.subs,
+            Content::Structure(FlatType::Func(
+                elem_decoder_var_slice,
+                this_decode_list_clos_var,
+                this_decode_list_ret_var,
+            )),
+        );
+
+        //   Decoder val  fmt -[uls]-> Decoder (List val) fmt | fmt has DecoderFormatting
+        // ~ Decoder elem fmt -a    -> b
+        env.unify(decode_list_fn_var, this_decode_list_fn_var);
+
+        let decode_list_member = AbilityMember(Symbol::DECODE_LIST, None, this_decode_list_fn_var);
+        let decode_list_fn = Box::new((
+            decode_list_fn_var,
+            Loc::at_zero(decode_list_member),
+            this_decode_list_clos_var,
+            this_decode_list_ret_var,
+        ));
+
+        let decode_list_call = Call(
+            decode_list_fn,
+            vec![(elem_decoder_var, Loc::at_zero(elem_decoder))],
+            CalledVia::Space,
+        );
+
+        (decode_list_call, this_decode_list_ret_var)
+    };
+
+    let bytes_sym = env.new_symbol("bytes");
+    let fmt_sym = env.new_symbol("fmt");
+    let fmt_var = env.subs.fresh_unnamed_flex_var();
+    let captures = vec![];
+
+    wrap_in_decode_custom_decode_with(
+        env,
+        bytes_sym,
+        (fmt_sym, fmt_var),
+        captures,
+        (decode_list_call, this_decode_list_ret_var),
+    )
+}