CODEWITHSUNDEEP
haskell
user668074
user668074

Reputation: 1141

Role of ASCII character variables in Data.Word8

I was trying to find a variable for the ord value of a letter (a=97,A=65) to make my code easier to read. I found that the Data.Word8 module defines some variables for characters in the following way:

_a = 0x61
_b = 0x62
_c = 0x63
_d = 0x64

etc.

However, when I try to use these variables I get the following error:

Found hole `_x' with type: t
Where: `t' is a rigid type variable bound by
           the inferred type of it :: t at <interactive>:27:1
Relevant bindings include it :: t (bound at <interactive>:27:1)
In the expression: _x
In an equation for `it': it = _x

This is the first time I've encountered typed-holes. After looking at an introduction to typed-holes I still don't understand (a) Why Data.Word8 uses them and (b) How can the typed-holes appear on the LHS of the equals sign. In the introduction I read they only appear on the RHS (c) How can I use these variables in my code?

If anyone has an explanation it would be appreciated.

Edit: I feel a bit stupid now. I was mixing up the Data.Word8 package with the Data.Word package which contains the Word8 data type.

Upvotes: 1

Views: 89

Answers (1)

Zeta
Zeta

Reputation: 105905

(a) Why Data.Word8 uses them

It doesn't. You forgot to import Data.Word8. Therefore, the identifier _x is unknown:

GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
ghci> _x

<interactive>:2:1:
    Found hole ‘_x’ with type: t
    Where: ‘t’ is a rigid type variable bound by
               the inferred type of it :: t at <interactive>:2:1
    Relevant bindings include it :: t (bound at <interactive>:2:1)
    In the expression: _x
    In an equation for ‘it’: it = _x
ghci> import Data.Word8
ghci> _x
120

(b) How can the typed-holes appear on the LHS of the equals sign

It does appear on the right hand side, because GHCi uses it = <last-expression>. Since the last expression was _x, you suddenly have a typed hole.

(c) How can I use these variables in my code?

Import Data.Word8.

Upvotes: 5

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