为什么GHCi没有显示与GHC相同的错误消息

Question

关于Haskell中给出的错误的推理编译时的类型错误我发现GHCi 有一个奇怪的行为

test.hs

members :: String -> Bool;
members str = and [ b | x <- str, b <- map (elem x) "abcde" ]

生成以下错误消息

Prelude> :l test.hs
[1 of 1] Compiling Main             ( test.hs, interpreted )

test.hs:2:53: error:
• Couldn't match type ‘Char’ with ‘t0 Char’
Expected type: [t0 Char]
Actual type: [Char]
• In the second argument of ‘map’, namely ‘"abcde"’
In the expression: map (elem x) "abcde"
In a stmt of a list comprehension: b <- map (elem x) "abcde"

而

GHCi

Prelude> members :: String -> Bool; members xs = and [ b | x <- xs, b <- map (elem x) "abcde"]

产生

<interactive>:18:78: error:
• Couldn't match type ‘Char’ with ‘[Char]’
Expected type: [[Char]]
Actual type: [Char]
• In the second argument of ‘map’, namely ‘"abcde"’
In the expression: map (elem x) "abcde"
In a stmt of a list comprehension: b <- map (elem x) "abcde"

更新

我还不明白的是，为什么GHC省略了Foldable t0类约束-I本以为错误消息是这样的：

test.hs:2:53: error:
• Couldn't match type ‘Char’ with ‘t0 Char’
Expected type: Foldable t0 => [t0 Char]
Actual type: [Char]
• In the second argument of ‘map’, namely ‘"abcde"’
In the expression: map (elem x) "abcde"
In a stmt of a list comprehension: b <- map (elem x) "abcde"

Answer 1

由FTP(可折叠遍历建议)函数elem引入得到了一个新的类型签名。

elem :: (Eq a, Foldable t) => a -> t a -> Bool

GHCi似乎使用了可以被停用的ExtendedDefaultRules扩展

:set -NoExtendedDefaultRules

这个扩展和trac:10971使得Foldable和Traversable默认为[]。

更新

类约束没有显示，因为它的类型检查是在稍后阶段完成的。