Saferish strings in Haskell APIs

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Safe strings in Haskell APIs

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Safe Safer strings in Haskell APIs

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Safe Safer Saferish strings in Haskell APIs

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Haskell • Purely functional • But pragmatically practical • Strongly typed • Pretty safe

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Strings

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OverloadedStrings • Added to GHC 6.8.1 • Allows string literal syntax to be used for other data types.

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OverloadedStrings

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OverloadedStrings

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So what? • Use custom datatypes instead of String at API boundaries. • If SQL commands aren’t strings, no string concatenation and less risk of injection vulnerabilities!

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mysql-simple • Database access library for MySQL written by Bryan O'Sullivan. • Uses this pattern to pass query text to the API.

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mysql-simple

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mysql-simple

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mysql-simple

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mysql-simple

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No concatenation

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No concatenation

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BUT! • All of the code which makes this work is public and you can import it. • So you (and any other developer) can circumvent this safety pretty trivially.

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So what do we want?

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How does this magic work? • Through the magic of compiler rules! • Allow library authors to specify rewrite rules to be applied during compilation. • Used for fusion and other optimisation techniques for data structure libraries. (text is very, very fast).

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String “Literals” • GHC generates a packed C-style string in the executable. • Code like this: ! • compiles to something like this:

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So rewrite this code!

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By default, it’s unsafe... It turns out that RULES do type inference. So we can write a RULE which replaces our fromStrings with out fromStringUnsafe.

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Unsafe calls

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Safe calls • The only safe calls are the ones which use static values!

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Identify and replace safe calls • Recall RULES use type inference. • And that the only safe calls are ones with static values. • And that static values have a speciﬁc type. • And are processed (before our code) with runtime support functions.

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Replace safe “unsafe” calls with “safe”

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Does it work?

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What’s next? • Make it less fragile. (Needs NOINLINE pragmas and is sensitive to optimisation). • Make the error compile time. (But it needs to be typesafe to work. #lolwut) • Probably much better and easier to write a GHC plugin.