Type checker cryptic erro on foldl

[anton-k]

 > foldl (\a as -> [a] ++ as) [] [1,2,3]
<repl>:1:17: Occurs error [a]

[anton-k]

ghci says that it can not construct infinite type here

[Shimuuar]

Nothing cryptic. Occurs check prevents construction of inifinite types

foldl ::  (b -> a -> b) -> b -> [a] -> b
(\a as -> [a] ++ as) :: c -> [c] -> [c]

thus unification foldl (\a as -> [a] ++ as) produces set of equations:

c ~ b 
a ~ [c]
b ~ [c]

From what we generate equation b ~ [b] which means b is infinite type. Occurs check prevents construction of such types

[anton-k]

Ok, but it's better write it out in this way.

[anton-k]

It's issue for better report of error of this type