if "read" can not infer type, we have to specify it

[wenshe...@gmail.com]

Prelude> read "300"

<interactive>:1:0:
Ambiguous type variable `a' in the constraint:
`Read a' arising from use of `read' at <interactive>:1:0-9
Probable fix: add a type signature that fixes these type
variable(s)
Prelude> read "300"::Int
300
Prelude>