Type of a double
haskell
Solution
This due to the dreaded monomorphism restriction. Basically, GHCi likes to choose default types when executed (the default `Fractional` type is `Double`), but when you ask the type using `:type` it chooses the most general version. You can disable this behavior with the `NoMonomorphismRestriction` extension:
> :set -XNoMonomorphismRestriction
> :set +t
> 0.15
0.15
it :: Fractional a => a
> :t 0.15
0.15 :: Fractional a => a
While this this extension has one of the scarier names, it's rather simple when you break it down:
Mono -> One
Morph -> shape (type)
ism -> thingy
Monomorphism -> one shape thingy -> one type thingy -> thing with a single type
So basically it's a really long word that means "single type". Then with "restriction", you get that the monomorphism restriction is restricting things to a single type. In this case, it's restricting numbers (the things) to the type `Double`. Without this restriction, the type of the numbers is only constrained by a type class, which can in theory be an infinite number of types.
Problem
Learning Haskell, in `ghci`: ``` Prelude Data.Ratio> :type 0.15 0.15 :: Fractional a => a Prelude Data.Ratio> 0.15 0.15 it :: Double ``` Why are types different? Are those two instances of 0.15 actually different types?