我正在尝试将一个简单的解释器从基于转换器的 monad 堆栈重写为基于 freer 的效果，但是我遇到了将我的意图传达给 GHC 类型系统的困难。
我目前只使用StateandFresh效果。我正在使用两种状态，我的效果跑步者看起来像这样：
runErlish g ls = run . runGlobal g . runGensym 0 . runLexicals ls
where runGlobal = flip runState
runGensym = flip runFresh'
runLexicals = flip runState
最重要的是，我定义了一个具有这种类型的函数 FindMacro：
findMacro :: Members [State (Global v w), State [Scope v w]] r
=> Arr r Text (Maybe (Macro (Term v w) v w))
到目前为止，所有这些都运行良好。当我尝试编写时，问题就来了macroexpand2（好吧，macroexpand1，但我正在简化它，所以这个问题更容易理解）：
macroexpand2 s =
do m <- findMacro s
return $ case m of
Just j -> True
Nothing -> False
这会产生以下错误：
Could not deduce (Data.Open.Union.Member'
(State [Scope v0 w0])
r
(Data.Open.Union.FindElem (State [Scope v0 w0]) r))
from the context (Data.Open.Union.Member'
(State [Scope v w])
r
(Data.Open.Union.FindElem (State [Scope v w]) r),
Data.Open.Union.Member'
(State (Global v w))
r
(Data.Open.Union.FindElem (State (Global v w)) r))
bound by the inferred type for `macroexpand2':
(Data.Open.Union.Member'
(State [Scope v w])
r
(Data.Open.Union.FindElem (State [Scope v w]) r),
Data.Open.Union.Member'
(State (Global v w))
r
(Data.Open.Union.FindElem (State (Global v w)) r)) =>
Text -> Eff r Bool
at /tmp/flycheck408QZt/Erlish.hs:(79,1)-(83,23)
The type variables `v0', `w0' are ambiguous
When checking that `macroexpand2' has the inferred type
macroexpand2 :: forall (r :: [* -> *]) v (w :: [* -> *]).
(Data.Open.Union.Member'
(State [Scope v w])
r
(Data.Open.Union.FindElem (State [Scope v w]) r),
Data.Open.Union.Member'
(State (Global v w))
r
(Data.Open.Union.FindElem (State (Global v w)) r)) =>
Text -> Eff r Bool
Probable cause: the inferred type is ambiguous
好的，我可以Members为类型添加注释：
macroexpand2 :: Members [State (Global v w), State [Scope v w]] r
=> Text -> Eff r Bool
现在我明白了：
Overlapping instances for Member (State [Scope v0 w0]) r
arising from a use of `findMacro'
Matching instances:
instance Data.Open.Union.Member'
t r (Data.Open.Union.FindElem t r) =>
Member t r
-- Defined in `Data.Open.Union'
There exists a (perhaps superclass) match:
from the context (Members
'[State (Global v w), State [Scope v w]] r)
bound by the type signature for
macroexpand2 :: Members
'[State (Global v w), State [Scope v w]] r =>
Text -> Eff r Bool
at /tmp/flycheck408QnV/Erlish.hs:(79,17)-(80,37)
(The choice depends on the instantiation of `r, v0, w0'
To pick the first instance above, use IncoherentInstances
when compiling the other instance declarations)
In a stmt of a 'do' block: m <- findMacro s
In the expression:
do { m <- findMacro s;
return
$ case m of {
Just j -> True
Nothing -> False } }
In an equation for `macroexpand2':
macroexpand2 s
= do { m <- findMacro s;
return
$ case m of {
Just j -> True
Nothing -> False } }
我被建议在 irc 上尝试forall r v w.，这没有任何区别。出于好奇，我IncoherentInstances在编译这段代码时尝试使用（我不想检查一个更自由的分支并玩），看看它是否会给我一个关于发生了什么的线索。它没有：
Could not deduce (Data.Open.Union.Member'
(State [Scope v0 w0])
r
(Data.Open.Union.FindElem (State [Scope v0 w0]) r))
arising from a use of `findMacro'
from the context (Members
'[State (Global v w), State [Scope v w]] r)
bound by the type signature for
macroexpand2 :: Members
'[State (Global v w), State [Scope v w]] r =>
Text -> Eff r Bool
at /tmp/flycheck408eru/Erlish.hs:(79,17)-(80,37)
The type variables `v0', `w0' are ambiguous
Relevant bindings include
macroexpand2 :: Text -> Eff r Bool
(bound at /tmp/flycheck408eru/Erlish.hs:81:1)
Note: there are several potential instances:
instance (r ~ (t' : r'), Data.Open.Union.Member' t r' n) =>
Data.Open.Union.Member' t r ('Data.Open.Union.S n)
-- Defined in `Data.Open.Union'
instance (r ~ (t : r')) =>
Data.Open.Union.Member' t r 'Data.Open.Union.Z
-- Defined in `Data.Open.Union'
In a stmt of a 'do' block: m <- findMacro s
In the expression:
do { m <- findMacro s;
return
$ case m of {
Just j -> True
Nothing -> False } }
In an equation for `macroexpand2':
macroexpand2 s
= do { m <- findMacro s;
return
$ case m of {
Just j -> True
Nothing -> False } }
所以，这就是我对 freer 内部结构的理解用尽的地方，我有疑问：
- 为什么会有重叠实例？我不明白这是从哪里来的。
- IncoherentInstances实际上做了什么？自动选择听起来很可能会导致难以调试的错误。
- 我如何在另一个函数中实际使用 findMacro？
干杯!