Int type conversion

Question

1) How do you convert from one Int type to a Num type?

Similar questions have been asked before and the answer has been (as is on the Haskell wiki) is to use fromIntegral. fromIntegral returns a Num type so I have to cast this to my desired format.

I need to take a Word16 and convert it to a Int64 so I am doing the following

let valueLength = (fromIntegral(tagLength) :: Int64)

where tagLength has type Word16

Is this approach correct?

2) How do you handle type conversion safely?

Coming from a Java background where there is for Integers I believe Short,Int and Long I can use a short as an int but not the other way around. In Haskell though if I write

256 :: Word8

in ghci it returns 0.

Answer 1

I need to take a Word16 and convert it to a Int64 so I am doing the following:

let valueLength = (fromIntegral(tagLength) :: Int64)

Is this approach correct?

Let's ask GHC!

Prelude Data.Word Data.Int> :t fromIntegral :: Word16 -> Int64
fromIntegral :: Word16 -> Int64 :: Word16 -> Int64

Looks good.

How do you handle type conversion safely?

Haskell does not have type conversion. At all. All "conversions" must be done by writing a function that "converts" from one type to another.

If I write 256 :: Word8 in ghci it returns 0.

Number literals are polymorphic. For things with no decimal point, fromInteger is used implicitly:

Prelude> :t 256
256 :: Num a => a
Prelude> :t fromInteger
fromInteger :: Num a => Integer -> a
Prelude> fromInteger (256 :: Integer) :: Word8
0

It might be nice if there was a warning or something for numeric literals with a monomorphic type that were out of range for that type; perhaps you should file a feature request on GHC's bug tracker.