Alexei Golovko wrote:
In Haskell 'do { e1; e2 }' expands to 'bind e1 (\_ -> e2) ' and does not fix
"return type" of 'e1'. This is usable, for example, with parser combinators, where
we often need call parser and discards its value. Another example --- IO action, which
returns some non-interesting technical details (as num of inserted rows or elapsed time).
In Ur, one need noisy 'ignore e1; e2' or (with unused free in 'e2' variable)
'_i<- e1; e2' if type 'e1' is not 'm {}'. Does this really help to prevent
errors? Which kinds? I am sure, the cost is too large.
Here's a simple example that motivates the design choice:
Version 1 of a library includes function [foo] of type [int ->
transaction {}]. The function doesn't involve any potential error
conditions worth considering in client code.
Version 2 of the library changes to [foo : int -> transaction result],
for some type [result] that includes both successful and error
outcomes. Client code will ignore the result, but really we want the
compiler to help us find all calls whose results now need to be analyzed
explicitly, to take appropriate action on error cases. If you look at a
piece of client code and decide that you don't care about errors there,
it's simple enough to wrap in [Monad.ignore]. The point is that you had
to think about each case.
At least, this difference should be documented in the reference manual: I have
thought that improperly declared monad for a some time.
I believe that part of the manual was already fine, but I've added a
note explicitly calling out the difference from Haskell. I also
realized that I hadn't defined the [()] syntax sugar earlier, and that
was probably the cause of the confusion; so I added that clarification
elsewhere. (These changes are in the public Mercurial repo only, ATM.)
PS. And also fix typo in chapter 4.1, Lexical conventions, of manual.pdf: in the
second row of table ASCII representation of long arrow should be '-->' but is
'->'
Weird. The "right" code was there in the source for LaTeX, which
apparently gets confused by two hyphens in a row. Fixed now. Thanks
for both reports!
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