[Haskell-cafe] Signature of a function

Stefan Holdermans sholderm at students.cs.uu.nl
Tue Jan 11 10:44:19 EST 2005


> Hint: Don't put signatures on functions, then. Instead, let the 
> compiler infer the type for you! If you want to know what the type is, 
> ask GHCi with :info. And if you think it is helpful documentation, you 
> can copy-paste the correct signature from ghci into your source code!

Although in this specific case you might be right, I'm not too sure if 
I agree with you. Especially for newbies, writing signatures upfront 
can be good practice.

First, it lets you think about *what* you are going to write before you 
actually write it. That way, it really forces structure on the way one 
codes, and lack of structure is what usually makes learning a new 
language harder.

Second, it gives you a way to test whether what you write is what you 
intended to write. Although the example is a bit contrived, I recently 
saw a piece of code written by a relative newbie for selecting the last 
element of a list:

   last (x : xs : y) = y

If she had written a type signature for this function, the compiler 
would have flagged this an error.

Admittedly, sometimes type signatures can get too intimidating for 
novice Haskellers. Think about polymorphism and type classes. But, 
well, one typically gets exposed to polymorphism quite soon, so that 
should be doable, while it's possible to stay ignorant of type classes 
for quite some while (if it weren't for those nasty error messages).

Just my two cents.



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