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Understanding systems

We learn what tutoring someone else has in common with learning anything: it's asking the right questions to figure out how things actually work, rather than how they seem to work, or how we've been taught they ought to work.

Full article (5–11 minute read): Understanding systems

Flashcard of the week

Skinner's book Effective Haskell is a great re-introduction to Haskell for someone like me, who has not used Haskell for many years and then suddenly get a job doing it. I learned a lot from it. One of the more basic things I learned the answer to the question

What is parametric polymorphism?

As an example of parametric polymorphism, we can use the for_ loop over lists, which has type signature

for_ :: [a] -> (a -> IO ()) -> IO ()

Another example is the fromMaybe function, which extracts a value from a Maybe, or returns a default value.

fromMaybe :: a -> Maybe a -> a

These both exhibit parametric polymorphism, but what does that mean?

Parametric polymorphism is when a function can accept values of many different types because the function itself never inspects the values.

In the case of for_, we supply a processing function which handles the actual elements of the list. That means for_ itself never has to deal with the elements, and thus it can loop over lists of any type.

In the case of fromMaybe, it never deals with the value because it only checks if it exists, and if it does not, the fromMaybe function returns whatever value we passed in. These values can be of any type, as long as the types agree, because fromMaybe doesn't care what the values are.

(This is in contrast to ad hoc polymorphism, which is when we support many different types by having a different implementation for each type. In Haskell, type classes are a way to implement ad hoc polymorphism.)

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