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functionhaskelltypesfunctional-programmingpattern-matching
user12596
user12596

Reputation: 81

Pattern matching on Ints

I am a beginner in learning Haskell, and I wanted to know if you could pattern match on Ints like so:

 add x 0 = x
 add x (1 + y) = 1 + x  + add x y,

Or maybe in this way:

 add x 0 = x
 add x (successor y) = 1 + x + add x y

Upvotes: 3

Views: 224

Answers (3)

Garrett M
Garrett M

Reputation: 103

The formatting of your code is a bit off so it is difficult to know what you're asking but you can using pattern matching for input of type Int. An example would be 

add x 0 = x
add x y = x + y

Upvotes: 0

isekaijin
isekaijin

Reputation: 19772

Pattern matching isn't arbitrary case analysis. It's a disciplined, but limited form of case analysis, where the cases are the constructors of a data type.

In the specific case of pattern matching integers, the constructors are taken to be the integer values. So you can use integer values as the cases for pattern-matching:

foo 0 = ...
foo 2 = ...
foo x = ...

But you can't use arbitrary expressions. The following code is illegal:

foo (2 * x) = ...
foo (2 * x + 1) = ...

You may know that ever integer is either of the form 2 * x or 2 * x + 1. But the type system doesn't know.

Upvotes: 2

mniip
mniip

Reputation: 796

There is an extension that lets you do that, but instead you should simply pattern match on y, and subtract 1 manually:

add x y = 1 + x + add x (y - 1)

The extension is called NPlusKPatterns. If you really want to use it (keep in mind it's deprecated in haskell 2010), it can be enabled by either passing a -XNPlusKPatterns parameter to GHC, or putting a {-# LANGUAGE NPlusKPatterns #-} at the top of your file.

Upvotes: 13

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