GitLab

This is a request for an adjustment of the syntax for the new recursive do notation. "rec { ... }" should parse/type check as an expression such that:

  mdo
      a <- getChar
      b <- f c
      c <- g b
      putChar c
      return b

can be written as

  do rec
      a <- getChar
      b <- f c
      c <- g b
      putChar c
      return b

at moment the closest you can get is ...

t5 =
  do rec   
      a <- getChar
      b <- f c
      c <- g b
      putChar c
    return b 

it seems rec { ... } is a binding construct not an expression and therefore can not be used in the final position (or sole component) of a do block

benefits

• drop in replacement for mdo, that is the construct "do rec"

becomes semantically and syntactically equivalent to the mdo notation

• current layout is maintained (which is much neater)
• I would argue that it is more intuitive

dificulty

• it seems to be a minor change?.

related

• this change seems to have been introduced as

http://hackage.haskell.org/trac/ghc/ticket/2798 somewhat surreptitiously (well, cought me by surprise, anyway.).

background

1. 12.1 introduced a change to the recursive do syntax

http://old.nabble.com/Update-on-GHC-6.12.1-td26103595.html

Instead of writing

  mdo
    a <- getChar
    b <- f c
    c <- g b
    putChar c
    return b

you would write

  do
    a <- getChar
    rec { b <- f c
          ; c <- g b }
    putChar c
    return b

A couple of issues about the change:

• the new syntax spoils layout (see above)
• migrating to the new syntax with the current limitation is

non-trivial, requires analysis of code (in some cases).

for the record ...

> t2 =
>  do rec
>       a <- getChar
>       b <- f c
>       c <- g b
>       putChar c
>       return b

> f = return . (const 'a')
> g = return

eg.lhs:23:6:
    The last statement in a 'do' construct must be an expression
Failed, modules loaded: none.

Trac metadata

Trac field	Value
Version	6.12.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	jvlask@hotmail.com
Operating system
Architecture