Yes. In Haskell, types are your friends. You should define new types liberally. I think the usual approach is to create newtype wrapper:
module EmailAddr (EmailAddr, mkEmailAddr) where
newtype EmailAddr = EmailAddr String
mkEmailAddr :: String -> Maybe EmailAddr mkEmailAddr str = if isEmailAddr then Just (EmailAddr str) else Nothing
The only way to make an EmailAddr is via the mkEmailAddr function, which checks that the string is actually a valid address (implementation omitted). Therefore, there's a guarantee that any EmailAddr is actually a well-formed email address, and any functions that operate on an EmailAddr can rely upon this. Of course, it might be useful to create an actual algebraic type instead:
data EmailAddr = EmailAddr { address :: String, domain :: String }
(and the domain could similarly be an algebraic type instead of a plain
string)
In general, you should be working with types that closely reflect the
domain you're working in. This will make your functions more clear, and
the compiler/type checker will be able to provide more help during
development.
John L.
On Sat, Mar 1, 2014 at 4:35 AM, KwangYul Seo
In Java, the Checker Framework ( http://types.cs.washington.edu/checker-framework/) provides a way to type check string literals. For example, Java signatures type system differentiates strings literals in different forms:
1. Unqualified strings : "Hello, world!" -> @Unqualified String
2. Fully qualified names: "package.Outer.Inner" -> @FullyQualifiedString String
3. Binary names: "package.Outer$Inner" -> @BinaryName String
4. Field descriptors: "Lpackage/Outer$Inner;" -> @FieldDescriptor String
It can do the similar checks with regular expressions or SQL statements.
Is it possible to type check string literals in Haskell? I think it would be nice if we can check if a given string literal is a valid URL or an email address at compile time.
Regards, Kwang Yul Seo
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