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Asked: 2020-12-17 19:14:20 +0800 CST 2020-12-17 19:14:20 +0800 CST

Why can't I perform this basic arithmetic?

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I am starting to learn Haskell with the book Learn You a Haskell for Great Good! , I come from using Python which is an imperative language. I am using the interpreter ghciand ran into the following:

Prelude> 5 * -3

<interactive>:2:1:    
    Precedence parsing error
        cannot mix `*' [infixl 7] and prefix `-' [infixl 6] in the same infix expression

From the error I understand that I cannot mix *with -in the expression, in the book they do not explain the reason why it does not work, they only recommend using parentheses:

Prelude> 5 * (-3)
-15

It occurs to me that it -is treated as a function and that is why when enclosing it in parentheses it is evaluated first before multiplying. So I tried the following:

Prelude> -(5)
-5
Prelude> +(5)

<interactive>:9:1: parse error on input `+'

Well now I'm confused. why is it not possible to multiply a negative number without the need to use parentheses?

haskell

1 Answers

  1. Best Answer

    The reason is: because the grammar does not allow it . Let me go into a little detail about this.

    Note: As you know, operators in haskell are functions in infix notation .

    Now let's see the error, it says:

    Precedence parsing error
    cannot mix `*' [infixl 7] and prefix `-' [infixl 6] in the same infix expression

    What does this mean? Well, to begin with, let's define what they mean by infixl 7e infixl 6, in their grammar there is something like this:

    infixr 9 .
infixr 8 ^, ^^, **
infixl 7 *, /, `quot`, `rem`, `div`, `mod`
infixl 6 +, -
infixr 5 :
infix 4 ==, /=, <, <=, >=, >
infixr 3 &&
infixr 2 ||
infixl 1 >>, >>=
infixr 1 =<<
infixr 0 $, $!, `seq`

    Note: You can see this here, in the prelude standard .

    What the error is telling you is that you can't mix elements infixl 7with infixl 6, that's the same reason you can't do something like this:

    Prelude> 5 * +3

    Since the syntax error is thrown ( you can try it here ):

    parse error on input `+'

    Wait, if it's a syntax error, why is the error above different?

    It turns out that el operador unario "-"is very special and is used by other parts of the grammar, so in theory this goes down a different branch of the syntax tree and fails elsewhere.

    If you want to read more about the latter you can see here (in English) .

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