This is generally considered to be good practice except when writing very short functions. From here on, we'll give all the functions that we make type declarations. Remember the list comprehension we made previously that filters a string so that only caps remain?
Here's how it looks like with a type declaration. That's because it takes one string as a parameter and returns another as a result. We didn't have to give this function a type declaration because the compiler can infer by itself that it's a function from a string to a string but we did anyway.
But how do we write out the type of a function that takes several parameters? Here's a simple function that takes three integers and adds them together:. The return type is the last item in the declaration and the parameters are the first three. If you want to give your function a type declaration but are unsure as to what it should be, you can always just write the function without it and then check it with :t. Functions are expressions too, so :t works on them without a problem.
Int stands for integer. It's used for whole numbers. Int is bounded, which means that it has a minimum and a maximum value. Usually on bit machines the maximum possible Int is and the minimum is Integer stands for, er … also integer. The main difference is that it's not bounded so it can be used to represent really really big numbers. I mean like really big. Int , however, is more efficient. Bool is a boolean type. It can have only two values: True and False.
Char represents a character. It's denoted by single quotes. A list of characters is a string. Tuples are types but they are dependent on their length as well as the types of their components, so there is theoretically an infinite number of tuple types, which is too many to cover in this tutorial. Note that the empty tuple is also a type which can only have a single value:.
What do you think is the type of the head function? Because head takes a list of any type and returns the first element, so what could it be? Let's check!
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What is this a? Is it a type? Remember that we previously stated that types are written in capital case, so it can't exactly be a type. Because it's not in capital case it's actually a type variable.
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That means that a can be of any type. This is much like generics in other languages, only in Haskell it's much more powerful because it allows us to easily write very general functions if they don't use any specific behavior of the types in them. Functions that have type variables are called polymorphic functions. The type declaration of head states that it takes a list of any type and returns one element of that type. Although type variables can have names longer than one character, we usually give them names of a, b, c, d …. Remember fst?
It returns the first component of a pair. Let's examine its type. We see that fst takes a tuple which contains two types and returns an element which is of the same type as the pair's first component.
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That's why we can use fst on a pair that contains any two types. Note that just because a and b are different type variables, they don't have to be different types. It just states that the first component's type and the return value's type are the same. A typeclass is a sort of interface that defines some behavior. If a type is a part of a typeclass, that means that it supports and implements the behavior the typeclass describes.
A lot of people coming from OOP get confused by typeclasses because they think they are like classes in object oriented languages. Well, they're not. You can think of them kind of as Java interfaces, only better. We can read the previous type declaration like this: the equality function takes any two values that are of the same type and returns a Bool.
The type of those two values must be a member of the Eq class this was the class constraint. The Eq typeclass provides an interface for testing for equality. Any type where it makes sense to test for equality between two values of that type should be a member of the Eq class. All standard Haskell types except for IO the type for dealing with input and output and functions are a part of the Eq typeclass.
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Eq is used for types that support equality testing. All the types we mentioned previously except for functions are part of Eq , so they can be tested for equality. All the types we covered so far except for functions are part of Ord. The compare function takes two Ord members of the same type and returns an ordering. To be a member of Ord , a type must first have membership in the prestigious and exclusive Eq club.
Members of Show can be presented as strings.
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All types covered so far except for functions are a part of Show. The most used function that deals with the Show typeclass is show. It takes a value whose type is a member of Show and presents it to us as a string. Read is sort of the opposite typeclass of Show. The read function takes a string and returns a type which is a member of Read. So far so good. Again, all types covered so far are in this typeclass. But what happens if we try to do just read "4"? For reducing the code length, I am only showing the changed code below.
You can see mypy is complaining about variable which does not have type annotations, and also found that in line 31, as argument 4 we are passing str , where as we were supposed to send in an Integer for the rull number. Let us fix these. Now, it does not complain about any error. You can see that in line 1, we imported Dict from the typing module.
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And, then using the same we added the type annotation of the self. We are saying that it is a dictionary which has string keys, and Integers as values. We also used our Student class as type of std1, std2, and std3 variables. You can learn more about types from PEP The typing module has detailed explanation and more examples about how to add type annotations in your codebase. You can also view the talk from Carl Meyer to learn about type checking in Python.
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