4 ms·

Casts don't only throw types away, they can also add type information to otherwise untyped input, such as reading data from a file or an HTTP request payload.

• dllthomas 12y ago
  Arguably, there is an intersection of "cast" and "conversion" where casts are valid.
• cbd1984 12y ago
  > Casts don't only throw types away, they can also add type information to otherwise untyped input, such as reading data from a file or an HTTP request payload. This is true. A more interesting example might be a program which creates database queries from user input casting input strings to the type "unescaped string", and then converting them to type "escaped string" by performing a string-escaping process on them. (A cast from "unescaped string" to "escaped string", which is done implicitly very often in languages where those types are not machine-checkable, is often disastrous.)
  • kyllo 12y ago
    Right, but I don't think this sort of type "conversion" (e.g. some function with type signature like this in Haskell) escapeQueryString :: ByteString -> Query is really adding or throwing away type information, it's just a function that accepts one type and returns another type. So it's not related to casting, except that it's the opposite of casting. But what that gives you is the ability to compose it with any other function that you define as accepting a `Query` type, and the program will refuse to compile if you ever accidentally try to pass it a ByteString. So it's about enforcing an invariant, in other words.