SO: What's the motivation to use a phantom type?

### The question

Don Stewart's Haskell in the Large's presentation mentioned Phantom Types:

data Ratio n = Ratio Double
1.234 :: Ratio D3

data Ask ccy = Ask Double
Ask 1.5123 :: Ask GBP

What's the motivation to use a phantom type?

### The answer

To answer the "what's the motivation to use a phantom type". There is two points:

• to make invalid states inrepresentable
• carry some of the information on the type level

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