[Haskell-cafe] An interesting curiosity
andrewcoppin at btinternet.com
Thu Sep 18 14:59:51 EDT 2008
I'm currently reading a C++ tutorial. (Don't ask!)
Of course, C++ is that crazy language where *assignment* is actually an
*operator*. Sick, sick people...
The tutorial pointed out that "x = 4" is a legal assignment, but "4 = x"
is not. "Obviously that would be silly."
Now it occurs to me... in Haskell, "4 = x" is actually a perfectly legal
thing to say. (!) It's pretty pointless, but it's legal. For example:
foo x = let 4 = x in 2*x
which, as best as I can tell, does exactly the same thing as (2*).
In a similar vein, one might write
foo x = do
let 4 = x
which also does more or less the same as (return).
However, consider the following:
foo :: Int -> Maybe Int
foo x = do
4 <- return x
This, it turns out, is an extremely odd way of checking whether x == 4.
All of this works of course because in Haskell, "=" is not an
assignment, it's a definition, and the RHS is not a variable, it's a
pattern. And "4" is a perfectly legitimate pattern. Now, if only I could
find a use for all this that borders on "useful"...! ;-)
More information about the Haskell-Cafe