feat: add sqlite support to starmelon interpreter

Cargo.toml

@@ -37,3 +37,11 @@ rusty_v8 = "0.32"
 futures = "0.3.15"
 serde_v8 = "0.15"
 
+# Required to add sql query support to interpreter. Because deno expects sync
+# ops to be synchronous, we have to use a second async executor to run the sqlx
+# functions. I read the code for oneshot
+# (https://github.com/faern/oneshot/commit/9aa237f185e1b65d61bf92c20350cf7bee0aa88b)
+# and it looks reasonable.
+sqlx = { version = "0.5", features = [ "sqlite", "macros", "runtime-tokio-rustls", "chrono", "json", "uuid" ] }
+oneshot = "0.1.3"
+

lib/sql/src/Astrid/Query.elm

@@ -1,8 +1,11 @@
 module Astrid.Query
     exposing
         ( Query
+        , errorToString
+        , execute
         , fetch
         , fetchOne
+        , fetchOptional
         , map
         , map2
         , map3
@@ -11,12 +14,54 @@ module Astrid.Query
         )
 
 import Json.Decode
+import Json.Encode
 import Array exposing (Array)
 
 {-| A value that knows how to load data from a SQL database.
 -}
 type Query a = Dummy
 
+type Error
+    = Execute String String
+    | Decode String Int Json.Decode.Error
+    | Failure String
+    | NotFound String
+
+errorToString : Error -> String
+errorToString error =
+    case error of
+        Execute sql message ->
+            "Execute `" ++ sql ++ "` failed `" ++ message ++ "`"
+
+        Decode sql index decodeError ->
+            "Decode results for `" ++ sql ++ "` failed at index `" ++ (String.fromInt index) ++ "` with error " ++ Json.Decode.errorToString(decodeError)
+
+        Failure message ->
+            "Failure `" ++ message ++ "`"
+
+        NotFound sql ->
+            "NotFound `" ++ sql ++ "`"
+
+execute : Query a -> Result Error a
+execute query =
+    dummyExecute
+
+{- Constructing the type inside this helper function will force Elm to generate
+the javascript of the Query type constructor above the execute function
+javascript.
+-}
+dummyExecute : Result Error a
+dummyExecute =
+    let
+        query = Dummy
+        _ = Execute "" ""
+        _ = Decode "" 0 (Json.Decode.Failure "" Json.Encode.null)
+        _ = Failure ""
+        _ = NotFound ""
+    in
+    Err (Failure "This package only works inside the starmelon interpreter")
+
+
 fetch : String -> List String -> Json.Decode.Decoder a -> Query (Array a)
 fetch sql parameters decoder =
     Dummy
@@ -25,6 +70,10 @@ fetchOne : String -> List String -> Json.Decode.Decoder a -> Query a
 fetchOne sql parameters decoder =
     Dummy
 
+fetchOptional : String -> List String -> Json.Decode.Decoder a -> Query (Maybe a)
+fetchOptional sql parameters decoder =
+    Dummy
+
 map : (a -> value) -> Query a -> Query value
 map f a =
     Dummy

src/elm.rs

@@ -69,7 +69,7 @@ pub fn load_interfaces(
                                 interfaces.insert(module_name, interface);
                             }
                             elmi::DependencyInterface::Private(package_name, unions, aliases) => {
-                                println!("skipping private interface {}", package_name);
+                                //println!("skipping private interface {}", package_name);
                                 //for (k, v) in unions {
                                 //    println!("  {}", k);
                                 //}

src/fixture.rs

@@ -21,8 +21,6 @@ pub(crate) fn generate<S: AsRef<str>>(
     buffer.push_str("-- START CUSTOMIZED PART\n");
     write!(buffer, "import {}\n\n", target_module.as_ref()).unwrap();
 
-    // if the input type is none then we don't have to generate an apply, and the port input type
-    // is ()
     match output {
         OutputType::String => buffer.push_str("encodeOutput = encodeString\n"),
         OutputType::Value => buffer.push_str("encodeOutput = encodeJson\n"),
@@ -30,6 +28,8 @@ pub(crate) fn generate<S: AsRef<str>>(
         OutputType::Html => buffer.push_str("encodeOutput = encodeHtml\n"),
     }
 
+    // if the input type is none then we don't have to generate an apply, and the port input type
+    // is ()
     match input {
         None => {
             buffer.push_str(&zero_arg_apply(

src/fixture/query.js

@@ -0,0 +1,306 @@
+// CORE QUERIES
+
+function __Debug_print(object) {
+    //Deno.core.print(JSON.stringify(object));
+    //Deno.core.print("\n");
+}
+
+function _Query_succeed(value)
+{
+	return {
+		$: 0,
+		a: value
+	};
+}
+
+
+var _Query_fetchOptional = F3(function(sql, args, decoder)
+{
+    return {
+        $: 1,
+        b: sql,
+        c: args,
+        d: { $: 0, a: decoder }
+    };
+});
+
+var _Query_fetchOne = F3(function(sql, args, decoder)
+{
+    return {
+        $: 2,
+        b: sql,
+        c: args,
+        d: { $: 1, a: decoder }
+    };
+});
+
+var _Query_fetchAll = F3(function(sql, args, decoder)
+{
+    return {
+        $: 3,
+        b: sql,
+        c: args,
+        d: { $: 2, a: decoder }
+    };
+});
+
+function _Query_mapMany(f, queries)
+{
+    return {
+        $: 4,
+        f: f,
+        g: queries
+    };
+}
+
+var _Query_andThen = F2(function(callback, query)
+{
+    return {
+        $: 5,
+        e: query,
+        h: callback
+    };
+});
+
+var _Query_map1 = F2(function(f, q1)
+{
+    return _Query_mapMany(f, [q1]);
+});
+
+var _Query_map2 = F3(function(f, q1, q2)
+{
+    return _Query_mapMany(f, [q1, q2]);
+});
+
+var _Query_map3 = F4(function(f, q1, q2, q3)
+{
+    return _Query_mapMany(f, [q1, q2, q3]);
+});
+
+var _Query_map4 = F5(function(f, q1, q2, q3, q4)
+{
+    return _Query_mapMany(f, [q1, q2, q3, q4]);
+});
+
+var _Query_map5 = F6(function(f, q1, q2, q3, q4, q5)
+{
+    return _Query_mapMany(f, [q1, q2, q3, q4, q5]);
+});
+
+var _Query_map6 = F7(function(f, q1, q2, q3, q4, q5, q6)
+{
+    return _Query_mapMany(f, [q1, q2, q3, q4, q5, q6]);
+});
+
+var _Query_map7 = F8(function(f, q1, q2, q3, q4, q5, q6, q7)
+{
+    return _Query_mapMany(f, [q1, q2, q3, q4, q5, q6, q7]);
+});
+
+var _Query_map8 = F9(function(f, q1, q2, q3, q4, q5, q6, q7, q8)
+{
+    return _Query_mapMany(f, [q1, q2, q3, q4, q5, q6, q7, q8]);
+});
+
+// RUN
+
+function _Query_runDecoder(decoder, sql, xs)
+{
+    switch (decoder.$) {
+        case 0:
+            if (xs.length === 0) {
+                return $elm$core$Result$Ok($elm$core$Maybe$Nothing);
+            }
+            var result = _Json_runOnString.f(decoder.a, xs[0]);
+
+            if (!$elm$core$Result$isOk(result))
+            {
+                return $elm$core$Result$Err(
+                    A3(
+                        $author$project$Astrid$Query$Decode,
+                        sql,
+                        0,
+                        result.a
+                    )
+                );
+            }
+            return $elm$core$Result$Ok($elm$core$Maybe$Just(result.a));
+
+        case 1:
+            if (xs.length === 0) {
+                return $elm$core$Result$Err($author$project$Astrid$Query$NotFound(sql));
+            }
+            var result = _Json_runOnString.f(decoder.a, xs[0]);
+
+            if (!$elm$core$Result$isOk(result))
+            {
+                return $elm$core$Result$Err(
+                    A3(
+                        $author$project$Astrid$Query$Decode,
+                        sql,
+                        0,
+                        result.a
+                    )
+                );
+            }
+            return result;
+
+        case 2:
+            var len = xs.length;
+            var array = new Array(len);
+            for (var i = 0; i < len; i++)
+            {
+                var string = xs[i];
+                try
+                {
+                    var value = JSON.parse(string);
+                    __Debug_print("parsed the json");
+                    __Debug_print(value);
+                    __Debug_print(decoder);
+                    var result = _Json_runHelp(decoder.a, value);
+                    __Debug_print("result of parsing the json");
+                    __Debug_print(result);
+                    if (!$elm$core$Result$isOk(result))
+                    {
+                        return $elm$core$Result$Err(
+                            A3(
+                                $author$project$Astrid$Query$Decode,
+                                sql,
+                                i,
+                                result.a
+                            )
+                        );
+                    }
+                    array[i] = result.a;
+                }
+                catch (e)
+                {
+                    return $elm$core$Result$Err(
+                        A3(
+                            $author$project$Astrid$Query$Decode,
+                            sql,
+                            i,
+                            A2(
+                                $elm$json$Json$Decode$Failure,
+                                'This is not valid JSON! ' + e.message, _Json_wrap(string)
+                            )
+                        )
+                    );
+                }
+            }
+	    return $elm$core$Result$Ok(_Json_toElmArray(array));
+    }
+}
+
+var _Query_execute = function(query)
+{
+    var queries = new Array;
+    var statements = new Array;
+    var decoders = new Array;
+    var values = new Array;
+    var callbacks = new Array;
+
+    queries.push(query);
+
+    while (true) {
+        var q;
+        while(q = queries.pop()) {
+            switch (q.$) {
+                case 0:
+                    values.push(q.a);
+                    break;
+
+                case 1:
+                case 2:
+                case 3:
+                    var moreThanOneRow = (q.$ === 3)
+                    var sql = q.b;
+                    var bindParameters = _List_toArray(q.c);
+                    var decoder = q.d;
+                    statements.push([moreThanOneRow, sql, bindParameters]);
+                    decoders.push(decoder);
+                    break;
+
+                case 4:
+                    callbacks.push({ $:'Map', a: q.f })
+                    // We know that the list of queries is limited to length 8,
+                    // which is much less then browser's stack overflow limits that
+                    // start around 100,000 elements.
+                    Array.prototype.push.apply(queries, q.g);
+                    break;
+
+                case 5:
+                    callbacks.push({ $:'AndThen', a: q.h })
+                    queries.push(q.e)
+                    break;
+            }
+        }
+
+        if (statements.length > 0) {
+            __Debug_print("statements = ");
+            __Debug_print(statements);
+            var queryResult = Deno.core.opSync(
+                'op_starmelon_batch_queries',
+                statements,
+            );
+            __Debug_print(queryResult);
+            // I am assuming here that the Rust code is serializing the same
+            // structure that the Elm compiler thinks we have.
+            if (!$elm$core$Result$isOk(queryResult)) {
+                return queryResult
+            }
+            var results = queryResult.a;
+            
+            var len = results.length;
+            for (var i = 0; i < len; i++) {
+                var result = _Query_runDecoder(decoders[i], statements[i][1], results[i])
+                if (!$elm$core$Result$isOk(result))
+                {
+                    return result
+                }
+                values.push(result.a);
+            }
+            statements.length = 0;
+            decoders.length = 0;
+        }
+
+        __Debug_print("processing callbacks stack");
+
+        reduce:
+        while(callbacks.length > 0) {
+            var last = callbacks[callbacks.length - 1];
+            switch (last.$) {
+                case 'Map':
+                    var arity = last.a.a;
+                    if (values.length < arity) {
+                        // This implies that queries.length > 0 because we must
+                        // have a way to generate the missing value(s) to call
+                        // this function.
+                        break reduce;
+                    }
+                    callbacks.pop();
+                    // Directly call the wrapped Elm function since we know all
+                    // the arguments
+                    var fun = last.a.f;
+                    var args = values.slice(-arity);
+                    values.length = values.length - args.length;
+                    values.push(Function.prototype.apply(fun, args));
+                    break;
+
+                case 'AndThen':
+                    callbacks.pop();
+                    var fun = last.a.f;
+                    // I think that if we have an AndThen then we will always have at least one value
+                    queries.push(fun(values.pop()));
+                    break;
+            }
+        }   
+
+        if (queries.length == 0 && callbacks.length == 0) {
+            // values must have one element in it.
+            return $elm$core$Result$Ok(values.pop())
+        }
+    }
+};
+
+var $author$project$Astrid$Query$execute = _Query_execute;

src/main.rs

@@ -1,10 +1,13 @@
 extern crate naive_wadler_prettier as pretty;
 use crate::reporting::{CompilerError, InterpreterError, Problem, SetupError, TypeError};
 use crate::timings::Timings;
+use deno_core::futures::StreamExt;
 use elm_project_utils::ChecksumConstraint;
 use os_pipe::dup_stderr;
 use pretty::pretty;
 use serde::{Deserialize, Serialize};
+use sqlx::sqlite::SqlitePool;
+use sqlx::Row;
 use std::cell::RefCell;
 use std::fs::{self, canonicalize};
 use std::hash::Hasher;
@@ -15,7 +18,7 @@ use std::sync::Arc;
 use std::time::Instant;
 use structopt::StructOpt;
 use tokio;
-use tracing::info_span;
+use tracing::{info_span, Instrument};
 
 mod elm;
 mod fixture;
@@ -23,6 +26,23 @@ mod reporting;
 mod timings;
 mod transpile;
 
+#[derive(Serialize)]
+#[serde(tag = "$")]
+pub(crate) enum ElmResult<T, E> {
+    Ok { a: T },
+    Err { a: E },
+}
+
+impl<T, E> ElmResult<T, E> {
+    fn ok(a: T) -> Self {
+        Self::Ok { a }
+    }
+
+    fn err(a: E) -> Self {
+        Self::Err { a }
+    }
+}
+
 fn main() {
     let args = Arguments::from_args();
 
@@ -38,11 +58,12 @@ fn main() {
             input,
             output,
             verbosity,
+            sqlite,
         } => {
             let start = Instant::now();
             let span = info_span!("exec");
             let timing_guard = span.enter();
-            let result = exec(file, debug, function, input, output, verbosity);
+            let result = exec(file, debug, function, input, output, verbosity, sqlite);
             drop(timing_guard);
             if let Err(problem) = result {
                 let span = info_span!("pretty print problem");
@@ -88,6 +109,7 @@ fn exec(
     input_source: Option<PathBuf>,
     output: Option<PathBuf>,
     verbosity: u64,
+    sqlite_path: Option<PathBuf>,
 ) -> Result<(), Problem> {
     // Our first elm make call is where we build the users program. There is a pretty good chance
     // this won't work.
@@ -219,7 +241,6 @@ fn exec(
     let final_script = (|| {
         let mut final_script = data
             .replace("'REPLACE_ME_WITH_JSON_STRINGIFY'", "JSON.stringify(x)")
-            //.replace("setTimeout", "(function(x, y) { globalThis.__bootstrap.timers.setTimeout(x, y)})")
             .replace(
                 "$elm$json$Json$Decode$fail('REPLACE_ME_WITH_BYTES_DECODER');",
                 r#" _Json_decodePrim(function(value) {
@@ -230,6 +251,35 @@ fn exec(
             )
             .replace(";}(this));", ";}(globalThis));");
 
+        if sqlite_path.is_some() {
+            final_script = final_script
+                .replace(
+                    "var $author$project$Astrid$Query$execute = function (query) {\n\treturn $author$project$Astrid$Query$dummyExecute;\n};",
+                    include_str!("fixture/query.js"),
+                )
+                .replace(
+                    "var $author$project$Astrid$Query$fetch = F3(\n\tfunction (sql, parameters, decoder) {\n\t\treturn $author$project$Astrid$Query$Dummy;\n\t});",
+                    "var $author$project$Astrid$Query$fetch = _Query_fetchAll;",
+                )
+                .replace(
+                    r#"var $author$project$Astrid$Query$map2 = {$: 'Dummy'};"#,
+                    r#"var $author$project$Astrid$Query$map2 = _Query_map2;"#,
+                );
+
+            // final_script.replace("var $author$project$Astrid$Query$run = ", "JSON.stringify(x)");
+            final_script.push_str("\n\n");
+            //final_script.push_str(r#"
+            //    Deno.core.print(JSON.stringify(
+            //        Deno.core.opSync(
+            //            'op_starmelon_batch_queries',
+            //            [ [true, "select json_object('id', id, 'foo', foo) from foobar", []]
+            //            , [false, "select json_object('id', id, 'foo', foo) from foobar", []]
+            //            ]
+            //        )
+            //    ))
+            //"#);
+        }
+
         final_script.push_str("\n\n");
         // I think that when I set this script to be the main module, I am skipping the
         // deno/runtime/js/99_main.js script that sets up a bunch of global variables. If I
@@ -237,11 +287,11 @@ fn exec(
         // NB. there are 706 lines of setup code that add a bunch of apis to the global window
         // scope. Figure out if I need to include all of them. For example, starmelon does not need
         // to perform http calls right now, but I eventually want to.
-        final_script.push_str("const { setTimeout } = globalThis.__bootstrap.timers;");
+        final_script.push_str("const { setTimeout } = globalThis.__bootstrap.timers;\n");
         final_script.push_str(
             "Deno.core.setMacrotaskCallback(globalThis.__bootstrap.timers.handleTimerMacrotask);\n",
         );
-        final_script.push_str("globalThis.setTimeout = setTimeout;");
+        final_script.push_str("globalThis.setTimeout = setTimeout;\n");
 
         final_script.push_str(&format!("var worker = Elm.{}.init();\n", &gen_module_name));
         // add a shortcut for invoking the function so I don't have to traverse so many object
@@ -249,7 +299,7 @@ fn exec(
         match input_type {
             None => {
                 final_script.push_str(
-                    "globalThis.runOnInput = function() { worker.ports.onInput.send(null)) };\n",
+                    "globalThis.runOnInput = function() { worker.ports.onInput.send(null) };\n",
                 );
             }
             Some(InputType::Value) => {
@@ -267,8 +317,7 @@ fn exec(
     globalThis.runOnInput = function(data) {
         const dv = new DataView(data.buffer)
         worker.ports.onInput.send(dv)
-    };
-    "#,
+    };"#,
                 );
             }
         }
@@ -326,6 +375,17 @@ fn exec(
     std::fs::write(&final_file, final_script)
         .map_err(|io_err| CompilerError::WriteOutputFailed(io_err, final_file.clone()))?;
 
+    // Create a tokio runtime before registering ops so we can block on futures inside sync ops
+    let span = info_span!("create tokio runtime");
+    let timing_guard = span.enter();
+    let sys = tokio::runtime::Builder::new_current_thread()
+        // The default number of additional threads for running blocking FnOnce is 512.
+        .max_blocking_threads(1)
+        .enable_all()
+        .build()
+        .unwrap();
+    drop(timing_guard);
+
     // step 10 create a v8 isolate. We need to register a different callback depending on
     // the output type (string, or bytes)
 
@@ -370,6 +430,107 @@ fn exec(
         }),
     );
 
+    // Step 10.B  setup the sqlite database feature
+    let sql_background_thread_handle = if let Some(database_url) = sqlite_path {
+        // I want to construct the connection in the initial thread so I can tell if the connection
+        // failed
+        let db_pool = sys
+            .block_on(async { SqlitePool::connect(&database_url.to_string_lossy()).await })
+            .unwrap();
+
+        let (worker_mailbox, rx) = std::sync::mpsc::channel::<(
+            oneshot::Sender<ElmResult<_, _>>,
+            Vec<(bool, String, Vec<String>)>,
+        )>();
+
+        let sql_worker_thread = std::thread::spawn(move || {
+            let worker = tokio::runtime::Builder::new_current_thread()
+                .enable_all()
+                .build()
+                .unwrap();
+            loop {
+                if let Ok((response, queries)) = rx.recv() {
+                    // I am not sure if I should only work on one database task at a time, or
+                    // submit as many takes as possible.  Just spawning the future onto this
+                    // exectutor does not seem to work, even though the docs say the thread pool
+                    // will poll the future until it completes.
+                    let db_pool_clone = db_pool.clone();
+                    let span = info_span!("inside sql queries futures");
+                    let f = async move {
+                        let start = Instant::now();
+                        let db_pool = db_pool_clone;
+                        let mut result: Vec<Vec<String>> = vec![];
+                        for (fetch_all, sql, _args) in queries {
+                            let mut acc = Vec::new();
+                            if fetch_all {
+                                let mut stream = sqlx::query(&sql).fetch(&db_pool);
+                                loop {
+                                    match stream.next().await {
+                                        None => break,
+                                        Some(Ok(row)) => {
+                                            match row.try_get::<String, _>(0) {
+                                                Ok(s) => acc.push(s),
+                                                // TODO set an error flag before returning this one
+                                                Err(_) => break,
+                                            };
+                                        }
+                                        Some(Err(err)) => {
+                                            eprintln!("got fetch_all sql error {:?}", err);
+                                            break;
+                                        }
+                                    }
+                                }
+                                result.push(acc);
+                            } else {
+                                let s = sqlx::query(&sql)
+                                    .fetch_one(&db_pool)
+                                    .await
+                                    .and_then(|row| row.try_get::<String, _>(0))
+                                    .unwrap();
+                                result.push(vec![s]);
+                            }
+                        }
+                        response.send(ElmResult::ok(result))
+                    };
+
+                    // I found it interesting that the runtime of the future from the viewpoint of
+                    // tracing was around 230us for a trivial select 2 rows query, but walltime I
+                    // measured was around 700us. So polling the future or waiting for file IO is
+                    // more expensive than I thought.
+                    worker.block_on(f.instrument(span));
+                } else {
+                    break;
+                }
+            }
+        });
+        let worker_mailbox_clone = worker_mailbox.clone();
+
+        worker.js_runtime.register_op(
+            "op_starmelon_batch_queries",
+            deno_core::op_sync(
+                move |_state, queries: Vec<(bool, String, Vec<String>)>, _: ()| {
+                    let worker_mailbox = worker_mailbox_clone.clone();
+                    let (sender, receiver) =
+                        oneshot::channel::<ElmResult<Vec<Vec<String>>, String>>();
+
+                    let span = info_span!("run sql");
+                    let timing_guard = span.enter();
+
+                    worker_mailbox.send((sender, queries)).unwrap();
+                    let elm_result = receiver.recv().unwrap();
+
+                    drop(timing_guard);
+
+                    Ok(elm_result)
+                },
+            ),
+        );
+
+        Some((worker_mailbox, sql_worker_thread))
+    } else {
+        None
+    };
+
     worker.js_runtime.sync_ops_cache();
 
     // step 11 marshal the input into the v8 isolate. If we are reading from an
@@ -411,15 +572,6 @@ fn exec(
         }
     };
 
-    let _start = Instant::now();
-    let span = info_span!("create tokio runtime");
-    let timing_guard = span.enter();
-    let sys = tokio::runtime::Builder::new_current_thread()
-        .enable_all()
-        .build()
-        .unwrap();
-    drop(timing_guard);
-
     let span = info_span!("eval javascript");
     let timing_guard = span.enter();
     sys.block_on(async move { runtime::xyz(worker, main_module, input).await })?;
@@ -451,6 +603,11 @@ fn exec(
         None => println!("nothing in the mailbox"),
     }
 
+    if let Some((tx, thread_handle)) = sql_background_thread_handle {
+        drop(tx);
+        thread_handle.join();
+    }
+
     Ok(())
 }
 
@@ -506,6 +663,8 @@ enum Arguments {
         output: Option<PathBuf>,
         #[structopt(short = "v", parse(from_occurrences))]
         verbosity: u64,
+        #[structopt(long)]
+        sqlite: Option<PathBuf>,
     },
     Transpile {
         #[structopt(parse(from_os_str))]
@@ -561,7 +720,12 @@ fn resolve_function_type(tipe: &elmi::Type) -> Result<(Option<InputType>, Output
 
             Err(TypeError::CantEvalType(tipe.clone()))
         }
-        elmi::Type::TType(_, _, _) => Err(TypeError::CantEvalCustomType),
+        elmi::Type::TType(module_name, name, args) => {
+            if module_name == "elm/virtual-dom/VirtualDom" && name == "Node" {
+                return Ok((None, OutputType::Html));
+            }
+            Err(TypeError::CantEvalCustomType)
+        }
         elmi::Type::TRecord(_, _) => Err(TypeError::CantEvalRecord),
         elmi::Type::TUnit => Err(TypeError::CantEvalUnit),
         elmi::Type::TTuple(_, _, _) => Err(TypeError::CantEvalTuple),
@@ -872,7 +1036,6 @@ mod runtime {
 
             let this = v8::undefined(scope).into();
 
-            let _start = Instant::now();
             let span = info_span!("dispatch v8 call");
             let timing_guard = span.enter();
             match input {

src/reporting.rs

@@ -123,7 +123,23 @@ impl From<rusty_v8::DataError> for InterpreterError {
 
 impl TypeError {
     pub fn to_doc(&self) -> Doc {
-        Doc::text("type error")
+        let mut title = "TYPE ERROR";
+        use TypeError::*;
+        let message = match self {
+            CantEvalRecord => Doc::text("cant eval record"),
+            CantEvalUnit => Doc::text("can't eval unit"),
+            CantEvalCustomType => reflow("can't eval custom type"),
+            _ => Doc::text("todo"),
+            // CantEvalHoleyAlias,
+            // CantEvalTuple,
+            // CantEvalGeneric,
+            // CantEvalType(elmi::Type),
+            // InputTypeNotSupported(elmi::Type),
+            // OutputTypeNotSupported(elmi::Type),
+            // EvalRequiresSingleArgument(elmi::Type),
+        };
+
+        vcat([to_message_bar(title, ""), Doc::text(""), message])
     }
 }