为什么 GHCi 不显示与 GHC 相同的错误消息

Question

对编译时 Haskell 类型错误中给出的错误进行推理，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"

然而

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

我也不明白的是，为什么 GHC 省略了Foldable t0类约束——我原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（Foldable-Traversable-Proposal）函数引入的elem获得了新的类型签名。

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

似乎 GHCi 使用了ExtendedDefaultRules扩展，它可以被停用

:set -NoExtendedDefaultRules

此扩展名和trac:10971使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。