Inferred type is not general enough

[Ketil Malde]

"Cagdas Ozgenc" <co19@cornell.edu> writes:

> The following function generates a type error.
> 
> headColumnwise :: [a] -> a
> headColumnwise line = [ head line | line <- p ]

Surely, the parameter should be "p"?

> ERROR ch6ex1.hs:14 - Inferred type is not general enough
> *** Expression    : headColumnwise
> *** Expected type : [a] -> a
> *** Inferred type : [[a]] -> [a]

> it compiles fine. Isn't [a]->a the most general type here?

If p is [a], then line (drawn from p) must be of type a.  But you do
"head line", which implies that line (and thus a) is a list.

-kzm
-- 
If I haven't seen further, it is by standing in the footprints of giants