``` 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. ```
Your guess is correct. The problem is, Haskell does not consider `a` in `myOrderFunc` and `[b]` in `myLen` equivalent. `a` means you feed the function any type, while `[b]` means it must be a list of values of the same type. So changing `a` to `[a]` woud eliminate the error.
Regards,
Qingbo Liu
On Nov 24, 2017, 16:33 -0500, Patrik Iselind
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?
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