为什么 GHCi 不显示与 GHC 相同的错误消息

Question

推理Haskell type error on compilation 中给出的错误我发现 GHCi 的一个奇怪行为

test.hs

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

产生以下错误信息

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

我也不明白，为什么 GHC 省略了 Foldable t0 类约束 - 我 本来期望错误消息类似于：

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(Foldable-Traversable-Proposal)功能引入elem有 获得了新的类型签名。

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

而且 GHCi 似乎使用了 ExtendedDefaultRules 扩展名，可以通过

:set -NoExtendedDefaultRules

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

更新

没有显示类约束，因为对它的类型检查是在稍后阶段完成的。