undefined | Better HN

0 pointsrapala13y ago0 comments

Associativity is very important in the case of IO. It guarantees that both

  (print a; print b;)print c;

and

  print a; (print b; print c;)

output abc.

0 comments

3 comments · 1 top-level

pacala13y ago· 2 in thread

Duh, blocks are evaluated in leftmost order. This is neither news, nor particularly profound in any number of popular curly brace languages. Rewritten with curly braces, so it's recognizable by most Blub programmers out there:

    { { print a; print b; } print c; }
    { print a; { print b; print c; } }

crntaylor13y ago

The distinction is not important in an eagerly evaluating language, which steps through its statements one by one and executes them as it finds them.

In a lazy, pure language it is of very great importance! With lazy evaluation you get no guarantees as to what order your expressions are evaluated in. Statements like "Blocks are evaluated in leftmost order" are just false in Haskell.

Monads give you the ability to sequence your I/O actions even though they are still lazily evaluated. This is why Haskell needs monads, and most other languages get by without them.

takeoutweight13y ago

Another way to think of it is that monads are embedded into the operational semantics of eager languages, so you that the programmer doesn't need to be aware of them. After all, monads were initially used to formally describe the behaviour of ML, an eager language, before they were used in Haskell.

j / k navigate · click thread line to collapse