manlen p1p2=abs(fst p1-fst p2)+abs(snd p1-snd p2)
points n = [(x,y)|x<-[-n..n], y<-[-n..n]]]
circle n = [p | p <-points n, manlen(0,0)p == n ]
I am not sure manlen(0,0)p==n in the last list inclusion notation.
Why do we take p==n for the manlen argument?
In haskel, infix operators that take arguments on both sides, such as ==, have lower priority than normal function calls.Therefore, manlen(0,0)p==n is interpreted as (manlen(0,0)p)==n instead of manlen(0,0)(p==n).This will return the Bool, so it will be a filter.Hence
mancircle n=[p|p<-points n, manlen(0,0)p==n]
returns only the manlen(0,0)p element where manlen(0,0)p is n.
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