Common Haskell error messages

Dealing with Errors

Since Haskell is a typed language, you’ll pretty quickly bump into type errors. Here’s an example of an error during a GHCi session:

Prelude> "string" ++ True

<interactive>:1:13: error:
    • Couldn't match expected type ‘[Char]’ with actual type ‘Bool’
    • In the second argument of ‘(++)’, namely ‘True’
        In the expression: "string" ++ True
        In an equation for ‘it’: it = "string" ++ True

This is the most common type error, “Couldn’t match expected type”. Even though the error looks long and scary, it’s pretty simple if you just read through it.

• The first line of the error message, <interactive>:1:13: error: tells us that the error occurred in GHCi. If we had loaded a file, we might instead get something like Sandbox.hs:3:17: error:, where Sandbox.hs is the name of the file, 3 is the line number and 17 is the number of a character in the line.

• The line • Couldn't match expected type ‘[Char]’ with actual type ‘Bool’ tells us that the immediate cause for the error is that there was an expression of type Bool, when GHCi was expecting to find an expression of type [Char]“. The location of this error was indicated in the first line of the error message. Note that the expected type is not always right. Giving type annotations by hand can help debugging typing errors.

• The line • In the second argument of ‘(++)’, namely ‘True’ tells that the expression that had the wrong type was the second argument of the operator (++). We’ll learn later why it’s surrounded by parentheses.

• The full expression with the error was "string" ++ True. As mentioned above, String is a type alias for [Char], the type of character lists. The first argument to ++ was a list of characters, and since ++ can only combine two lists of the same type, the second argument should’ve been of type [Char] too.

• The line In an equation for ‘it’: it = "string" ++ True says that the expression occurred in the definition of the variable it, which is a default variable name that GHCi uses for standalone expressions. If we had a line x = "string" ++ True in a file, or a declaration let x = "string" ++ True in GHCi, GHCi would print In an equation for ‘x’: x = "string" ++ True instead.

There are also others types of errors.

Prelude> True + 1

<interactive>:6:1: error:
    • No instance for (Num Bool) arising from a use of ‘+’
    • In the expression: True + 1
        In an equation for ‘it’: it = True + 1

This is the kind of error you get when you try to use a numeric function like + on something that’s not a number.

The hardest error to track down is usually this:

    Prelude> True +

    <interactive>:10:7: error:
        parse error (possibly incorrect indentation or mismatched brackets)

There are many ways to cause it. Probably you’re missing some characters somewhere. We discuss Indendation in the end of part 15.

No instance for (Eq a) arising from a use of ‘==’

You’ve probably tried to use x==Nothing to check if a value is Nothing. Use pattern matching instead. The reason for this error is that values of type Maybe a can’t be compared because Haskell doesn’t know how to compare values of the polymorphic type a. You’ll find more about this in the next lecture. Use pattern matching instead of == for now.