```
exercises.hs:33:13:
    Couldn't match expected type ‘[b0]’ with actual type ‘a’
      ‘a’ is a rigid type variable bound by
          the type signature for myOrderFunc :: a -> a -> Ordering
          at exercises.hs:31:16
    Relevant bindings include
      y :: a (bound at exercises.hs:32:15)
      x :: a (bound at exercises.hs:32:13)
      myOrderFunc :: a -> a -> Ordering (bound at exercises.hs:32:1)
    In the first argument of ‘myLen’, namely ‘x’
    In the first argument of ‘(<)’, namely ‘myLen x’
Failed, modules loaded: none.
```

Den 2017-11-24 kl. 20:04, skrev Quentin Liu:

Yes, you could pass the function a list of strings as well. A string is just a list of Chars. The type signature `a` does not restrict the range of types you could pass to the function.

 

That seem strange to me. Wouldn't that mean that i could write the declaration of myOrderFunc as `myOrderFunc :: a -> a -> Ordering` as well? GHCI give me an error on this though so obviously it's wrong. I just don't see why. Why cannot a represent [b]?

Could you copy and paste the error message here?

Sure, the error i get follows
```
exercises.hs:33:13:
    Couldn't match expected type ‘[b0]’ with actual type ‘a’
      ‘a’ is a rigid type variable bound by
          the type signature for myOrderFunc :: a -> a -> Ordering
          at exercises.hs:31:16
    Relevant bindings include
      y :: a (bound at exercises.hs:32:15)
      x :: a (bound at exercises.hs:32:13)
      myOrderFunc :: a -> a -> Ordering (bound at exercises.hs:32:1)
    In the first argument of ‘myLen’, namely ‘x’
    In the first argument of ‘(<)’, namely ‘myLen x’
Failed, modules loaded: none.
```
Attaching the updated exercises.hs for reference.

I'm still not very good at interpreting Haskell's error messages, they are quite cryptic to me. My interpretation/guess of the above is that my `a` is too 'wide' or how you express it. Haskell seem to expect some form of list. Most likely since i want a length and lists are perhaps everything in Haskell that can produce a length. I've hardly scratched the surface of what i imagine is Haskell so i cannot say anything for sure yet.

The way I use to think about type signature is, when you trying to substitute type variables such as `a`, substitute it into a concrete type that you are working with.

I'm having a hard time understanding your way of thinking about type signatures. Could you perhaps elaborate a bit more on it?

// Patrik
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