Type signature
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.