Hi, if im calling (on the console) "typeOf 1" after importing Data.Typeable, i will gain "Integer", as i expected, but if i do so in code which is to compile, then there raises an error ------------ import Data.Typeable main= print (typeOf 1) ------------ $ ghc -c DT.hs $ ghc -c DT.hs DT.hs:4:15: Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `1' at DT.hs:4:15 `Typeable t' arising from a use of `typeOf' at DT.hs:4:8-15 Probable fix: add a type signature that fixes these type variable(s) what can i do, to solve this problem? Regards
On 2009 Jan 11, at 5:27, Frank Schwidom wrote:
"Integer", as i expected, but if i do so in code which is to compile, then there raises an error
Yes. Numeric literals like 5 are actually read as (fromIntegral 5); then ghci applies defaulting to reduce it to Integer. When compiled, you must resolve the polymorphism yourself. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH
As Brandon explained, since 1 is polymorphic, and print is polymorphic in its argument, the compiler doesn't know which type to choose. You could fix it, for example, like this:
------------ import Data.Typeable
main= print (typeOf (1 :: Integer)) ------------
You might protest, what is the point of using typeOf if you have to actually put in the type annotation anyway? Well, this is a rather contrived example; usually in a real program GHC would be able to infer the type and an annotation would be unnecessary. -Brent
participants (3)
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Brandon S. Allbery KF8NH -
Brent Yorgey -
Frank Schwidom