Re: [Haskell-cafe] Syntax for modifying nested product types

Mark Carroll writes:

For fairly flat things, the updates using field labels are okay. However, when I have a function that needs to tweak various bits of various "deep" fields, based on various conditions, then it gets really ugly, not least because of the conditions: foo { bar = baz } becomes,

let old = bar foo in foo { bar = if cond then f old else old }

or whatever. Do I need such a let for each component and sub-component of the object that has a child that may be modified, or will

foo { bar = if cond then f (bar foo) else bar foo }

be suitably optimised?

If the pattern is very common, how about just naming it? perhaps cond f field = if cond then f field else field foo { bar = perhaps cond0 f (bar foo) , wib = perhaps cond1 g (wib foo) } Regards, Malcolm