roc-lang/roc
1
1
Fork 0
mirror of https://github.com/roc-lang/roc.git synced 2024-09-22 08:17:40 +03:00
Code Issues Packages Projects Releases Wiki Activity

Merge pull request #2372 from rtfeldman/effect-forever

Browse Source 
Add `Effect.forever`
This commit is contained in:
Folkert de Vries 2022-01-20 09:16:32 +01:00 committed by GitHub
commit 82c7e8c37e
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
5 changed files with 864 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

787
compiler/load/src/effect_module.rs
View File

@ -21,15 +21,21 @@ type Builder = for<'r, 's, 't0, 't1> fn(
&'t1 mut VarStore,
) -> (Symbol, Def);
pub const BUILTIN_EFFECT_FUNCTIONS: [(&str, Builder); 3] = [
pub const BUILTIN_EFFECT_FUNCTIONS: &[(&str, Builder)] = &[
// Effect.after : Effect a, (a -> Effect b) -> Effect b
("after", build_effect_after),
// Effect.map : Effect a, (a -> b) -> Effect b
("map", build_effect_map),
// Effect.always : a -> Effect a
("always", build_effect_always),
// Effect.forever : Effect a -> Effect b
("forever", build_effect_forever),
// Effect.loop : a, (a -> Effect [ Step a, Done b ]) -> Effect b
("loop", build_effect_loop),
];
const RECURSIVE_BUILTIN_EFFECT_FUNCTIONS: &[&str] = &["forever", "loop"];
// the Effects alias & associated functions
//
// A platform can define an Effect type in its header. It can have an arbitrary name
@ -52,7 +58,7 @@ pub fn build_effect_builtins(
exposed_symbols: &mut MutSet<Symbol>,
declarations: &mut Vec<Declaration>,
) {
for (_, f) in BUILTIN_EFFECT_FUNCTIONS.iter() {
for (name, f) in BUILTIN_EFFECT_FUNCTIONS.iter() {
let (symbol, def) = f(
env,
scope,
@ -62,8 +68,34 @@ pub fn build_effect_builtins(
);
exposed_symbols.insert(symbol);
declarations.push(Declaration::Declare(def));
let is_recursive = RECURSIVE_BUILTIN_EFFECT_FUNCTIONS.iter().any(|n| n == name);
if is_recursive {
declarations.push(Declaration::DeclareRec(vec![def]));
} else {
declarations.push(Declaration::Declare(def));
}
}
// Useful when working on functions in this module. By default symbols that we named do now
// show up with their name. We have to register them like below to make the names show up in
// debug prints
if false {
env.home.register_debug_idents(&env.ident_ids);
}
}
macro_rules! new_symbol {
($scope:expr, $env:expr, $name:expr) => {{
$scope
.introduce(
$name.into(),
&$env.exposed_ident_ids,
&mut $env.ident_ids,
Region::zero(),
)
.unwrap()
}};
}
fn build_effect_always(
@ -588,6 +620,755 @@ fn build_effect_after(
(after_symbol, def)
}
/// turn `value` into `@Effect \{} -> value`
fn wrap_in_effect_thunk(
body: Expr,
effect_tag_name: TagName,
closure_name: Symbol,
captured_symbols: Vec<Symbol>,
var_store: &mut VarStore,
) -> Expr {
let captured_symbols: Vec<_> = captured_symbols
.into_iter()
.map(|x| (x, var_store.fresh()))
.collect();
// \{} -> body
let const_closure = {
let arguments = vec![(
var_store.fresh(),
Loc::at_zero(empty_record_pattern(var_store)),
)];
Expr::Closure(ClosureData {
function_type: var_store.fresh(),
closure_type: var_store.fresh(),
closure_ext_var: var_store.fresh(),
return_type: var_store.fresh(),
name: closure_name,
// captured_symbols: vec![(value_symbol, var_store.fresh())],
captured_symbols,
recursive: Recursive::NotRecursive,
arguments,
loc_body: Box::new(Loc::at_zero(body)),
})
};
// `@Effect \{} -> value`
Expr::Tag {
variant_var: var_store.fresh(),
ext_var: var_store.fresh(),
name: effect_tag_name,
arguments: vec![(var_store.fresh(), Loc::at_zero(const_closure))],
}
}
/// given `effect : Effect a`, unwrap the thunk and force it, giving a value of type `a`
fn force_effect(
effect: Expr,
effect_tag_name: TagName,
thunk_symbol: Symbol,
var_store: &mut VarStore,
) -> Expr {
let whole_var = var_store.fresh();
let ext_var = var_store.fresh();
let thunk_var = var_store.fresh();
let pattern = Pattern::AppliedTag {
ext_var,
whole_var,
tag_name: effect_tag_name,
arguments: vec![(thunk_var, Loc::at_zero(Pattern::Identifier(thunk_symbol)))],
};
let pattern_vars = SendMap::default();
// pattern_vars.insert(thunk_symbol, thunk_var);
let def = Def {
loc_pattern: Loc::at_zero(pattern),
loc_expr: Loc::at_zero(effect),
expr_var: var_store.fresh(),
pattern_vars,
annotation: None,
};
let ret_var = var_store.fresh();
let force_thunk_call = {
let boxed = (
var_store.fresh(),
Loc::at_zero(Expr::Var(thunk_symbol)),
var_store.fresh(),
ret_var,
);
let arguments = vec![(var_store.fresh(), Loc::at_zero(Expr::EmptyRecord))];
let call = Expr::Call(Box::new(boxed), arguments, CalledVia::Space);
Loc::at_zero(call)
};
Expr::LetNonRec(Box::new(def), Box::new(force_thunk_call), var_store.fresh())
}
fn build_effect_forever(
env: &mut Env,
scope: &mut Scope,
effect_symbol: Symbol,
effect_tag_name: TagName,
var_store: &mut VarStore,
) -> (Symbol, Def) {
// morally
//
// Effect.forever = \effect -> Effect.after effect (\_ -> Effect.forever effect)
//
// Here we inline the `Effect.after`, and get
//
// Effect.forever : Effect a -> Effect b
// Effect.forever = \effect ->
// @Effect \{} ->
// @Effect thunk1 = effect
// _ = thunk1 {}
// @Effect thunk2 = Effect.forever effect
// thunk2 {}
//
// We then rely on our defunctionalization to turn this into a tail-recursive loop.
// First the `@Effect` wrapper melts away
//
// Effect.forever : ({} -> a) -> ({} -> b)
// Effect.forever = \effect ->
// \{} ->
// thunk1 = effect
// _ = thunk1 {}
// thunk2 = Effect.forever effect
// thunk2 {}
//
// Then we defunctionalize
//
// foreverInner = \{}, { effect } ->
// thunk1 = effect
// _ = thunk1 {}
// thunk2 = Effect.forever effect
// thunk2 {}
//
// Effect.forever : [ C foreverInner { effect : T } ]
// Effect.forever = \effect ->
// C { effect }
//
// And we have to adjust the call
//
// foreverInner = \{}, { effect } ->
// thunk1 = effect
// _ = thunk1 {}
// thunk2 = Effect.forever effect
// when thunk2 is
// C env -> foreverInner {} env.effect
//
// Making `foreverInner` perfectly tail-call optimizable
let forever_symbol = {
scope
.introduce(
"forever".into(),
&env.exposed_ident_ids,
&mut env.ident_ids,
Region::zero(),
)
.unwrap()
};
let effect = {
scope
.introduce(
"effect".into(),
&env.exposed_ident_ids,
&mut env.ident_ids,
Region::zero(),
)
.unwrap()
};
let body = build_effect_forever_body(
env,
scope,
effect_tag_name.clone(),
forever_symbol,
effect,
var_store,
);
let arguments = vec![(var_store.fresh(), Loc::at_zero(Pattern::Identifier(effect)))];
let function_var = var_store.fresh();
let after_closure = Expr::Closure(ClosureData {
function_type: var_store.fresh(),
closure_type: var_store.fresh(),
closure_ext_var: var_store.fresh(),
return_type: var_store.fresh(),
name: forever_symbol,
captured_symbols: Vec::new(),
recursive: Recursive::Recursive,
arguments,
loc_body: Box::new(Loc::at_zero(body)),
});
let mut introduced_variables = IntroducedVariables::default();
let signature = {
let var_a = var_store.fresh();
let var_b = var_store.fresh();
introduced_variables.insert_named("a".into(), var_a);
introduced_variables.insert_named("b".into(), var_b);
let effect_a = build_effect_alias(
effect_symbol,
effect_tag_name.clone(),
"a",
var_a,
Type::Variable(var_a),
var_store,
&mut introduced_variables,
);
let effect_b = build_effect_alias(
effect_symbol,
effect_tag_name,
"b",
var_b,
Type::Variable(var_b),
var_store,
&mut introduced_variables,
);
let closure_var = var_store.fresh();
introduced_variables.insert_wildcard(closure_var);
Type::Function(
vec![effect_a],
Box::new(Type::Variable(closure_var)),
Box::new(effect_b),
)
};
let def_annotation = roc_can::def::Annotation {
signature,
introduced_variables,
aliases: SendMap::default(),
region: Region::zero(),
};
let pattern = Pattern::Identifier(forever_symbol);
let mut pattern_vars = SendMap::default();
pattern_vars.insert(forever_symbol, function_var);
let def = Def {
loc_pattern: Loc::at_zero(pattern),
loc_expr: Loc::at_zero(after_closure),
expr_var: function_var,
pattern_vars,
annotation: Some(def_annotation),
};
(forever_symbol, def)
}
fn build_effect_forever_body(
env: &mut Env,
scope: &mut Scope,
effect_tag_name: TagName,
forever_symbol: Symbol,
effect: Symbol,
var_store: &mut VarStore,
) -> Expr {
let closure_name = {
scope
.introduce(
"forever_inner".into(),
&env.exposed_ident_ids,
&mut env.ident_ids,
Region::zero(),
)
.unwrap()
};
let inner_body = build_effect_forever_inner_body(
env,
scope,
effect_tag_name.clone(),
forever_symbol,
effect,
var_store,
);
let captured_symbols = vec![effect];
wrap_in_effect_thunk(
inner_body,
effect_tag_name,
closure_name,
captured_symbols,
var_store,
)
}
fn build_effect_forever_inner_body(
env: &mut Env,
scope: &mut Scope,
effect_tag_name: TagName,
forever_symbol: Symbol,
effect: Symbol,
var_store: &mut VarStore,
) -> Expr {
let thunk1_symbol = {
scope
.introduce(
"thunk1".into(),
&env.exposed_ident_ids,
&mut env.ident_ids,
Region::zero(),
)
.unwrap()
};
let thunk2_symbol = {
scope
.introduce(
"thunk2".into(),
&env.exposed_ident_ids,
&mut env.ident_ids,
Region::zero(),
)
.unwrap()
};
// Effect thunk1 = effect
let thunk_from_effect = {
let whole_var = var_store.fresh();
let ext_var = var_store.fresh();
let thunk_var = var_store.fresh();
let pattern = Pattern::AppliedTag {
ext_var,
whole_var,
tag_name: effect_tag_name.clone(),
arguments: vec![(thunk_var, Loc::at_zero(Pattern::Identifier(thunk1_symbol)))],
};
let pattern_vars = SendMap::default();
Def {
loc_pattern: Loc::at_zero(pattern),
loc_expr: Loc::at_zero(Expr::Var(effect)),
expr_var: var_store.fresh(),
pattern_vars,
annotation: None,
}
};
// thunk1 {}
let force_thunk_call = {
let ret_var = var_store.fresh();
let boxed = (
var_store.fresh(),
Loc::at_zero(Expr::Var(thunk1_symbol)),
var_store.fresh(),
ret_var,
);
let arguments = vec![(var_store.fresh(), Loc::at_zero(Expr::EmptyRecord))];
let call = Expr::Call(Box::new(boxed), arguments, CalledVia::Space);
Loc::at_zero(call)
};
// _ = thunk1 {}
let force_thunk1 = Def {
loc_pattern: Loc::at_zero(Pattern::Underscore),
loc_expr: force_thunk_call,
expr_var: var_store.fresh(),
pattern_vars: Default::default(),
annotation: None,
};
// recursive call `forever effect`
let forever_effect = {
let boxed = (
var_store.fresh(),
Loc::at_zero(Expr::Var(forever_symbol)),
var_store.fresh(),
var_store.fresh(),
);
let arguments = vec![(var_store.fresh(), Loc::at_zero(Expr::Var(effect)))];
Expr::Call(Box::new(boxed), arguments, CalledVia::Space)
};
// ```
// Effect thunk2 = forever effect
// thunk2 {}
// ```
let force_thunk2 = Loc::at_zero(force_effect(
forever_effect,
effect_tag_name,
thunk2_symbol,
var_store,
));
Expr::LetNonRec(
Box::new(thunk_from_effect),
Box::new(Loc::at_zero(Expr::LetNonRec(
Box::new(force_thunk1),
Box::new(force_thunk2),
var_store.fresh(),
))),
var_store.fresh(),
)
}
fn build_effect_loop(
env: &mut Env,
scope: &mut Scope,
effect_symbol: Symbol,
effect_tag_name: TagName,
var_store: &mut VarStore,
) -> (Symbol, Def) {
let loop_symbol = new_symbol!(scope, env, "loop");
let state_symbol = new_symbol!(scope, env, "state");
let step_symbol = new_symbol!(scope, env, "step");
let body = build_effect_loop_body(
env,
scope,
effect_tag_name.clone(),
loop_symbol,
state_symbol,
step_symbol,
var_store,
);
let arguments = vec![
(
var_store.fresh(),
Loc::at_zero(Pattern::Identifier(state_symbol)),
),
(
var_store.fresh(),
Loc::at_zero(Pattern::Identifier(step_symbol)),
),
];
let function_var = var_store.fresh();
let after_closure = Expr::Closure(ClosureData {
function_type: var_store.fresh(),
closure_type: var_store.fresh(),
closure_ext_var: var_store.fresh(),
return_type: var_store.fresh(),
name: loop_symbol,
captured_symbols: Vec::new(),
recursive: Recursive::Recursive,
arguments,
loc_body: Box::new(Loc::at_zero(body)),
});
let mut introduced_variables = IntroducedVariables::default();
let signature = {
let var_a = var_store.fresh();
let var_b = var_store.fresh();
introduced_variables.insert_named("a".into(), var_a);
introduced_variables.insert_named("b".into(), var_b);
let effect_b = build_effect_alias(
effect_symbol,
effect_tag_name.clone(),
"b",
var_b,
Type::Variable(var_b),
var_store,
&mut introduced_variables,
);
let state_type = {
let step_tag_name = TagName::Global("Step".into());
let done_tag_name = TagName::Global("Done".into());
Type::TagUnion(
vec![
(step_tag_name, vec![Type::Variable(var_a)]),
(done_tag_name, vec![Type::Variable(var_b)]),
],
Box::new(Type::EmptyTagUnion),
)
};
let effect_state_type = {
let closure_var = var_store.fresh();
introduced_variables.insert_wildcard(closure_var);
let actual = {
Type::TagUnion(
vec![(
effect_tag_name,
vec![Type::Function(
vec![Type::EmptyRec],
Box::new(Type::Variable(closure_var)),
Box::new(state_type.clone()),
)],
)],
Box::new(Type::EmptyTagUnion),
)
};
Type::Alias {
symbol: effect_symbol,
type_arguments: vec![("a".into(), state_type)],
lambda_set_variables: vec![roc_types::types::LambdaSet(Type::Variable(
closure_var,
))],
actual: Box::new(actual),
}
};
let closure_var = var_store.fresh();
introduced_variables.insert_wildcard(closure_var);
let step_type = Type::Function(
vec![Type::Variable(var_a)],
Box::new(Type::Variable(closure_var)),
Box::new(effect_state_type),
);
let closure_var = var_store.fresh();
introduced_variables.insert_wildcard(closure_var);
Type::Function(
vec![Type::Variable(var_a), step_type],
Box::new(Type::Variable(closure_var)),
Box::new(effect_b),
)
};
let def_annotation = roc_can::def::Annotation {
signature,
introduced_variables,
aliases: SendMap::default(),
region: Region::zero(),
};
let pattern = Pattern::Identifier(loop_symbol);
let mut pattern_vars = SendMap::default();
pattern_vars.insert(loop_symbol, function_var);
let def = Def {
loc_pattern: Loc::at_zero(pattern),
loc_expr: Loc::at_zero(after_closure),
expr_var: function_var,
pattern_vars,
annotation: Some(def_annotation),
};
(loop_symbol, def)
}
fn build_effect_loop_body(
env: &mut Env,
scope: &mut Scope,
effect_tag_name: TagName,
loop_symbol: Symbol,
state_symbol: Symbol,
step_symbol: Symbol,
var_store: &mut VarStore,
) -> Expr {
let closure_name = {
scope
.introduce(
"loop_inner".into(),
&env.exposed_ident_ids,
&mut env.ident_ids,
Region::zero(),
)
.unwrap()
};
let inner_body = build_effect_loop_inner_body(
env,
scope,
effect_tag_name.clone(),
loop_symbol,
state_symbol,
step_symbol,
var_store,
);
let captured_symbols = vec![state_symbol, step_symbol];
wrap_in_effect_thunk(
inner_body,
effect_tag_name,
closure_name,
captured_symbols,
var_store,
)
}
fn applied_tag_pattern(
tag_name: TagName,
argument_symbols: &[Symbol],
var_store: &mut VarStore,
) -> Pattern {
let arguments = argument_symbols
.iter()
.map(|s| {
let pattern = Pattern::Identifier(*s);
(var_store.fresh(), Loc::at_zero(pattern))
})
.collect();
Pattern::AppliedTag {
ext_var: var_store.fresh(),
whole_var: var_store.fresh(),
tag_name,
arguments,
}
}
fn build_effect_loop_inner_body(
env: &mut Env,
scope: &mut Scope,
effect_tag_name: TagName,
loop_symbol: Symbol,
state_symbol: Symbol,
step_symbol: Symbol,
var_store: &mut VarStore,
) -> Expr {
let thunk1_symbol = new_symbol!(scope, env, "thunk3");
let thunk2_symbol = new_symbol!(scope, env, "thunk4");
let new_state_symbol = new_symbol!(scope, env, "newState");
let done_symbol = new_symbol!(scope, env, "done");
// Effect thunk1 = step state
let thunk_from_effect = {
let whole_var = var_store.fresh();
let ext_var = var_store.fresh();
let thunk_var = var_store.fresh();
let pattern = Pattern::AppliedTag {
ext_var,
whole_var,
tag_name: effect_tag_name.clone(),
arguments: vec![(thunk_var, Loc::at_zero(Pattern::Identifier(thunk1_symbol)))],
};
let pattern_vars = SendMap::default();
// `step state`
let rhs = {
let boxed = (
var_store.fresh(),
Loc::at_zero(Expr::Var(step_symbol)),
var_store.fresh(),
var_store.fresh(),
);
let arguments = vec![(var_store.fresh(), Loc::at_zero(Expr::Var(state_symbol)))];
Expr::Call(Box::new(boxed), arguments, CalledVia::Space)
};
Def {
loc_pattern: Loc::at_zero(pattern),
loc_expr: Loc::at_zero(rhs),
expr_var: var_store.fresh(),
pattern_vars,
annotation: None,
}
};
// thunk1 {}
let force_thunk_call = {
let ret_var = var_store.fresh();
let boxed = (
var_store.fresh(),
Loc::at_zero(Expr::Var(thunk1_symbol)),
var_store.fresh(),
ret_var,
);
let arguments = vec![(var_store.fresh(), Loc::at_zero(Expr::EmptyRecord))];
let call = Expr::Call(Box::new(boxed), arguments, CalledVia::Space);
Loc::at_zero(call)
};
// recursive call `loop newState step`
let loop_new_state_step = {
let boxed = (
var_store.fresh(),
Loc::at_zero(Expr::Var(loop_symbol)),
var_store.fresh(),
var_store.fresh(),
);
let arguments = vec![
(var_store.fresh(), Loc::at_zero(Expr::Var(new_state_symbol))),
(var_store.fresh(), Loc::at_zero(Expr::Var(step_symbol))),
];
Expr::Call(Box::new(boxed), arguments, CalledVia::Space)
};
// ```
// Effect thunk2 = loop effect
// thunk2 {}
// ```
let force_thunk2 = force_effect(
loop_new_state_step,
effect_tag_name,
thunk2_symbol,
var_store,
);
let step_branch = {
let step_tag_name = TagName::Global("Step".into());
let step_pattern = applied_tag_pattern(step_tag_name, &[new_state_symbol], var_store);
roc_can::expr::WhenBranch {
patterns: vec![Loc::at_zero(step_pattern)],
value: Loc::at_zero(force_thunk2),
guard: None,
}
};
let done_branch = {
let done_tag_name = TagName::Global("Done".into());
let done_pattern = applied_tag_pattern(done_tag_name, &[done_symbol], var_store);
roc_can::expr::WhenBranch {
patterns: vec![Loc::at_zero(done_pattern)],
value: Loc::at_zero(Expr::Var(done_symbol)),
guard: None,
}
};
let branches = vec![step_branch, done_branch];
let match_on_force_thunk1 = Expr::When {
cond_var: var_store.fresh(),
expr_var: var_store.fresh(),
region: Region::zero(),
loc_cond: Box::new(force_thunk_call),
branches,
};
Expr::LetNonRec(
Box::new(thunk_from_effect),
Box::new(Loc::at_zero(match_on_force_thunk1)),
var_store.fresh(),
)
}
pub fn build_host_exposed_def(
env: &mut Env,
scope: &mut Scope,

10
compiler/mono/src/ir.rs
View File

@ -738,15 +738,19 @@ impl<'a> Procs<'a> {
ret_var: Variable,
layout_cache: &mut LayoutCache<'a>,
) -> Result<ProcLayout<'a>, RuntimeError> {
// anonymous functions cannot reference themselves, therefore cannot be tail-recursive
let is_self_recursive = false;
let raw_layout = layout_cache
.raw_from_var(env.arena, annotation, env.subs)
.unwrap_or_else(|err| panic!("TODO turn fn_var into a RuntimeError {:?}", err));
let top_level = ProcLayout::from_raw(env.arena, raw_layout);
// anonymous functions cannot reference themselves, therefore cannot be tail-recursive
// EXCEPT when the closure conversion makes it tail-recursive.
let is_self_recursive = match top_level.arguments.last() {
Some(Layout::LambdaSet(lambda_set)) => lambda_set.contains(symbol),
_ => false,
};
match patterns_to_when(env, layout_cache, loc_args, ret_var, loc_body) {
Ok((_, pattern_symbols, body)) => {
// an anonymous closure. These will always be specialized already

5
compiler/mono/src/layout.rs
View File

@ -569,6 +569,11 @@ impl<'a> LambdaSet<'a> {
*self.representation
}
/// Does the lambda set contain the given symbol?
pub fn contains(&self, symbol: Symbol) -> bool {
self.set.iter().any(|(s, _)| *s == symbol)
}
pub fn is_represented(&self) -> Option<Layout<'a>> {
if let Layout::Struct(&[]) = self.representation {
None

35
examples/benchmarks/platform/Task.roc
View File

@ -1,9 +1,42 @@
interface Task
exposes [ Task, succeed, fail, after, map, putLine, putInt, getInt ]
exposes [ Task, succeed, fail, after, map, putLine, putInt, getInt, forever, loop ]
imports [ fx.Effect ]
Task ok err : Effect.Effect (Result ok err)
forever : Task val err -> Task * err
forever = \task ->
looper = \{ } ->
task
|> Effect.map
\res ->
when res is
Ok _ ->
Step {}
Err e ->
Done (Err e)
Effect.loop {} looper
loop : state, (state -> Task [ Step state, Done done ] err) -> Task done err
loop = \state, step ->
looper = \current ->
step current
|> Effect.map
\res ->
when res is
Ok (Step newState) ->
Step newState
Ok (Done result) ->
Done (Ok result)
Err e ->
Done (Err e)
Effect.loop state looper
succeed : val -> Task val *
succeed = \val ->
Effect.always (Ok val)

35
examples/cli/platform/Task.roc
View File

@ -1,9 +1,42 @@
interface Task
exposes [ Task, succeed, fail, await, map, onFail, attempt ]
exposes [ Task, succeed, fail, await, map, onFail, attempt, forever, loop ]
imports [ fx.Effect ]
Task ok err : Effect.Effect (Result ok err)
forever : Task val err -> Task * err
forever = \task ->
looper = \{ } ->
task
|> Effect.map
\res ->
when res is
Ok _ ->
Step {}
Err e ->
Done (Err e)
Effect.loop {} looper
loop : state, (state -> Task [ Step state, Done done ] err) -> Task done err
loop = \state, step ->
looper = \current ->
step current
|> Effect.map
\res ->
when res is
Ok (Step newState) ->
Step newState
Ok (Done result) ->
Done (Ok result)
Err e ->
Done (Err e)
Effect.loop state looper
succeed : val -> Task val *
succeed = \val ->
Effect.always (Ok val)