How to specify type for bulk list assignments

Question

I have the following code that looks like this

[a,b,c,d] = ["a","b","c","d"]

The compiler reports the warning:

Warning: Definition but no type signature for 'a'
         Inferred type: a :: [Char]

How to silence the warning and specify the type for this expression?

Answer 1

You can add an explicit type signature for the variables:

a, b, c, d :: String
[a,b,c,d] = ["a","b","c","d"]

There should also be some compiler flag that disables the warning since it's common practice to not explicitly specify the types of every helper variable.

Answer 2

No errors in GHCi:

Prelude> let [a,b,c,d] = ["a","b","c","d"]

If you want to give the entire fragment a type, try,

   Prelude> let x :: [String]
                     x@[a,b,c,d] = ["a","b","c","d"]