AllowAmbiguousTypes与扩展程序一起使用时，扩展程序有多危险TypeApplications？
GHC 手册给出了以下模棱两可类型的示例：
class C a
f :: C a => Int
f = 3
AllowAmbiguousTypes不使用以下消息时无法编译：
file.hs:8:6: error:
• Could not deduce (C a0)
from the context: C a
bound by the type signature for:
f :: forall a. C a => Int
at file.hs:8:6-15
The type variable ‘a0’ is ambiguous
• In the ambiguity check for ‘f’
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
In the type signature: f :: C a => Int
|
8 | f :: C a => Int
| ^^^^^^^^^^
使用AllowAmbiguousTypes，它可以正确编译。
但是，即使使用以下示例也无法编译AllowAmbiguousTypes：
class C a
f :: C a => Int
f = 3
g :: C a => Int
g = f
尝试编译时出现以下错误：
file.hs:12:5: error:
• Could not deduce (C a0) arising from a use of ‘f’
from the context: C a
bound by the type signature for:
g :: forall a. C a => Int
at file.hs:11:1-15
The type variable ‘a0’ is ambiguous
• In the expression: f
In an equation for ‘g’: g = f
|
12 | g = f
| ^
这可以通过启用TypeApplications和编写它来编写，如下所示：
class C a
f :: C a => Int
f = 3
g :: forall a. C a => Int
g = f @a
AllowAmbiguousTypes听起来很吓人，但根据 GHC 手册中的描述，它似乎相当温和，尤其是与TypeApplications.
这不会AllowAmbiguousTypes导致运行时错误，对吧？
AllowAmbiguousTypes和的这种组合TypeApplications似乎也用于一些非常流行的包中，比如约束。