I am new in Haskell, I noticed # has specific role when I was reading a code, someone knows what does exactly do?
# is used in pattern matching for keeping the binding to the whole thing.
Example:
In x#(a:as), x will refer to the entire list a:as.
In the above example, you can get the whole list via x instead of typing yourself a:as again.
Related
I am writing documentation for a library using haddock, and, for reasons that are not really necessary to explain, I need to include a little code block in my documentation that looks like:
z(<>)
Importantly there can be no space between z and (<>). It may be a bit esoteric but
z (<>)
would make my documentation incorrect, even if it is more stylistically correct.
Now I believe that hyperlinks to both z and (<>) would be helpful. Neither has a very informative name, so a link that helps people remember their definitions and purpose is nice.
So my code without the hyperlinks looks like:
#z(<>)#
And to add hyperlinks I just use single quotes:
#'z''(<>)'#
Except that doesn't work, haddock sees 'z'' and thinks that I mean to link z' (a function that does exist in my module), and then just leaves the rest alone. The rendered output looks like
z'(<>)'
Now as an experiment I deleted the definition of z', however the only difference this makes is that the link to z' goes away. The raw text remains the same. The next thing I tried was ditching #s altogether and did
'z''(<>)'
Which also created a hyperlink to z' and left the rest untouched, the same problem as earlier except now nothing is in a code block.
How can I make a code block that links two functions without a space between?
You can separate the two functions into different code blocks. If there is no space between the code blocks, it will appear no different than a single code block. So
#'z'##'(<>)'#
will render as desired.
You can also do it in one code block by moving the 's inside of the parentheses to only surround <>.
#'z'('<>')#
This will render slightly differently with the parentheses not being part of any hyperlink, however this may be desired.
Here is an alternative solution to add to the answer you already provided:
You can mix and match ' and `. These two will also be rendered correctly by haddock:
-- | #`z`'(<>)'#
-- | #'z'`(<>)`#
At the same time I've tried your solution #'z'##'(<>)'# and for some reason it did not render for me properly, but with haddock you never know.
Here are all of the ones that I've tried:
-- * #'z'##'(<>)'#
-- * #'z'('<>')#
-- * #'z'`(<>)`#
-- * #`z`'(<>)'#
With corresponding result:
When working with lists in Haskell, i can simply load my file into ghci and type head list or last list to get the information that I need. But if I have a list of lists, lets say: list = [[1,2,3],[4,5,6]], how can I get the first element (head) of the first list (in this case, 1), or the last element of the second list (in this case, 6), and so one?
If all you need is the first or last element, concat will flatten the list for you.
There is an indexing function (!!), so for your examples, head . (!!0) and last . (!!1) . If your question is more general, then please elaborate. Indexing is not great because it can throw errors if you attempt to index past the end of the list, so usually we try to work around that, eg. by saying "well I want to do the same thing to every element of the list so I don't really need the index" (map function) or "if I really do need the index then don't use it directly") (zip [0..], or use of eg. a record data type).
Also, Hoogle is your friend if you've not met it before. If you can break down your functions into simple ones you think might be standard, then search their type signatures, that's usually a good place to start. Hoogle [a] -> Int -> a even if you don't find exactly what you want, often if you find something similar and browse it's module or source code you can find something helpful.
I have an assignment, similar to scrabble. I have to check if a subset is in the set. can only use a letter once. so if subset has 2t and the set has 1t it is false.
My problem is, I used 2 inputs to allow people to enter the subset and set, but that create a string no breaks between the letters which mean split or list won't create a LIST with individual letters. (at least I can't find any way.)
My plan was something like
wordset = word.lower().split()
subset = letters.lower()
for i in range(len(subset)):
if i in subset and in set:
set.remove(i)
I know that properly won't work but until I can get it into a list or someone gives me a hint how to do it with string I can't start testing it. Sorry for so much writing.
If you wish to get a list of characters in a given string you can use a list comprehension:
characters = [x for x in some_string]
I would like to change the behavior of menhir's output in follwoing way:
I want it to look up all grammatical alternatives if it finds any, and put them in a list and get me back this ambigouus interpretation. It shall not reduce conflicts, just store them.
In the source code of menhir, it seems to me, that I have to look in "Engine.ml". The resultant syntactically determined token comes in a variant type item "Accepted v" as a state of a checkpoint of the grammatical automaton. This content is found by a function "accept env prod" before, that is part of a bundle of recursive functions, that change the states.
Do you have a tip, how I could change these functions to put all the possible results in the list here and proceed as if nothing happened? Or do you think, that this wont work anyway?
Thanks.
What you are looking for is a GLR parser generator (G is for generalized). Menhir is not such tool, and I doubt you could modify it easily to do what you want.
However, there is another tool that does exactly what you want: dypgen.
I am wondering if it is possible to keep assigned values in ghci when a module is reloaded?
For example i assign a value in ghci:
ghci> let x = 1
or
ghci> x <- getLine
After entering :r to reload an existing module x is not in scope anymore. Is it generally possible to keep the assignment available, like for example in the Python interpreter? (this is really convenient...)
Even tho that actually each line in ghci represents a function that is (monadically) bound to the next one I am still wondering if maintaining that state is possible.
I'm not aware of any way of doing this.
The trouble is that you could have some variable bound to a value of a certain type, edit the source to remove that type, and hit reload. Now you have a variable of a type that no longer exists.
Still, you would think it shouldn't be too hard to detect that, and discard just the variables that don't make sense any more. (The really fun part is presumably when a type still exists but has a different number of fields now, or something like that...)