如何检查haskell中的类型

Question

I don't know how to check type of variable in haskell, Here i mean , when i read something from console with getLine ,however i expect it to be an interger but user can enter a string also,then i don't want my program to crash. For example if someone inputs a string and i try to convert it to Int then it will crash(exception) so i want to check whether it is convertable or not. How do i do that ? Thanks for any help :)

 main1 = do
        let g <- getLine
            k = g :: Int 
            if(k :: Int)
                then ........ 

Answer 1

EDIT: Notice you always have a string from getLine - that's the type it returns. If that string contains an ascii representation of a number then great and keep reading.

If you have a string, g , and say g :: Int the compiler will simply so "no, you are wrong, that's a String". You need to perform a translation - parse the string and compute an Int. The most readily available methods are read in the Prelude and readMaybe in Text.Read .

Read will work but throws exceptions on invalid input:

Prelude> read "4742" :: Int
4742
Prelude> read "no" :: Int
*** Exception: Prelude.read: no parse
Prelude> read "191andmore"
*** Exception: Prelude.read: no parse

The maybe variant is exception safe:

Prelude> import Text.Read
Prelude Text.Read> readMaybe "181" :: Maybe Int
Just 181
Prelude Text.Read> readMaybe "no" :: Maybe Int
Nothing
Prelude Text.Read> readMaybe "211andmore" :: Maybe Int
Nothing