Re: = vs ->

On 10 Oct 2001, Ketil Malde wrote:

Mark Carroll writes:

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On Tue, 9 Oct 2001, Ashley Yakeley wrote:

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At 2001-10-09 11:55, Mark Carroll wrote:

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What is the rationale for when Haskell demands a "=" and when it demands a "->"?

Okay, I can't give you anything formal, but here's my intuitive understanding of things

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e.g.

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x :: Integer -> Integer

A function "from" and Integer to an Integer. Even more obvious if you have one more parameter:

g :: Integer -> Integer -> Integer

g takes an Integer and returns a function that takes an Integer and returns an Integer. Equals-assignment would be very non-intuitive here.

As I understand it, the equals sign is used whenever the item on both sides are equal, i.e., one side can be replaced with the other without changing meaning. Of course, in the case of a function definition it's the degenerate equality you get from defining the lhs in terms of the rhs. The -> is used whenever you've got something on the right that `leads to' to something on the left, eg case x of Maybe y ->True Nothing ->False It is not the case that `Maybe y' is the same as True, so = is clearly inappropraite. Likewise for lambdas (\x->x+2 doesn't have x = x+2). It's perhaps less clear because after using functional languages for any length of time you get very used to thinking of function definitions as a restricted kind of rewrite rule, and rewrite rules may not necessarily have any connection to a notion of equality. ___cheers,_dave________________________________________________________ www.cs.bris.ac.uk/~tweed/pi.htm |tweed's law: however many computers email: tweed@cs.bris.ac.uk | you have, half your time is spent work tel: (0117) 954-5250 | waiting for compilations to finish.