[Haskell-cafe] Is there a null statement that does nothing?
inforichland at gmail.com
Wed Oct 21 20:49:14 EDT 2009
Yes, an if statement must have both 'then' and 'else' branches. As an
example, what if you had
let a = if b == 2 then True else False
and you were missing an else branch? What would 'a' get assigned to?
The if statement "returns" a value so must have both branches.
However, in a monadic constraint, there are the functions 'when' and
'unless.' They allow conditional evaluation of expressions in a monadic
context. For example,
main = do
line <- getLine
when (line == "hello") putStrLn "Hello back!"
On Wed, Oct 21, 2009 at 7:43 PM, michael rice <nowgate at yahoo.com> wrote:
> It looks like both the THEN and the ELSE in an IF expression must each have
> an expression. What's a graceful way to do nothing in either or both slots,
> kind of like the Fortran CONTINUE statement.
> [michael at localhost ~]$ ghci
> GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer ... linking ... done.
> Loading package base ... linking ... done.
> Prelude> if (1==1) then else
> <interactive>:1:15: parse error on input `else'
> Prelude> if (1==1) then True else
> <interactive>:1:24: parse error (possibly incorrect indentation)
> Prelude> if (1==1) then True else False
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