[Haskell-cafe] Type checking string literal in Haskell?

[Niklas Haas]

On Sat, 1 Mar 2014 21:35:09 +0900, KwangYul Seo <kwangyul.seo at gmail.com> wrote:
> 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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I think QuasiQuoters were designed for pretty much this purpose. They're
basically functions from Strings to expressions (or
types/declarations/patterns, depending on the context).

A quasiquoter like [url|http://example.com] could parse the string
passed to it ("http://example.com") at compile time and return an
expression of type URL (or throw an error or something else), or maybe
even some other data structure that will end up being useful.