Generalizing State and Maybe

Now that you’ve spent some time building general abstractions for working with state and failing computations we want to go back through and look for commonalities.

First, go back through all the code you wrote for Set 1, Find all the generalized functions and copy their type signatures (not the code) into a new file. You can skip the functions that solved special case exercises. Then do the same thing for Set 2 and copy those type signatures into the same file.

Now look at these type signatures and look for functions from Set 1 that have a similar type signature pattern to functions from Set 2. The type signatures obviously won’t be the same, but there are similar patterns in both sets. You should come up with two pairs of function equivalences.

Here is the hex encoded answer. But again, spend some effort trying to see this correspondence for yourself.

67656E54776F207E3D206C696E6B2C2067656E6572616C42207E3D20794C696E6B

Now that you have the correspondences, for each pair write a more general type signature that works for both the Gen version and the Maybe version. Think back to things we’ve done before. If two signatures are exactly the same except for one difference, replace that difference with a type variable. It doesn’t matter what letter you use for the type variable as long as it is different from all the other type variables that occur in the signature. But just to make your life easier we’ll give you a hint: use the letter m. Here’s the hex-encoded answer:

67656E54776F2C206C696E6B0A20203A3A206D2061202D3E202861202D3E206D206229202D3E206D20620A0A67656E6572616C422C20794C696E6B0A20203A3A202861202D3E2062202D3E206329202D3E206D2061202D3E206D2062202D3E206D2063

You won’t be able to implement these functions yet. We’re just writing type signatures right now and looking at the patterns involved.

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