Type signature

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A type signature is a line like

inc :: Num a => a -> a

that tells, what is the type of a variable. In the example inc is the variable, Num a => is the context and a -> a is its type, namely a function type with the kind * -> *.

A very simple example looks like this

title :: String

which restricts the variable title to the the type String. Binding a value of any other type will lead to a type missmatch. For example binding 42 to title by writing title = 42 will lead to an error looking like this in ghc 7.10

    No instance for (Num String) arising from the literal 42
    In the expression: 42
    In an equation for title: title = 42

To better understand the error message, take a look at the types, in ghci you can use the `:t ` command which will show you the type for a given expression

>:t title
title :: String
>:t 42
42 :: Num a => a

If instead declaring the types like in this example

title :: String
value :: Integer

value = 42
title = value

the error message becomes clearer

    Couldn`t match type Integer with [Char]
    Expected type: String
      Actual type: Integer
    In the expression: value
    In an equation for title: title = value

but there is still room for confusion because the first lines mentiones the type `[Char]` which does not appear in the type signatures in the example. This comes form the fact that String is just a renaming for [Char]. The compiler only typechecks the expressions after resolving the renaming.

It is considered good style to add a type signature to every top-level variable.

References