can you express that grammar in ebnf? I think the parser would need to be at least ll(C) where C is bigger than 1.

Would the presence of $_ denote a lambda? how would nested lambda's work?

Why not:

    put 1 2 3 4 5 | filter -> > $1 2 | map -> * 2 $1

I think that would be easier to parse, and not much typing overhead, also $_ is a weird thing.

This is equivalent to the following bash incantation?:

    cat <<eof\
     | while read a;do if [[ $a -gt 2 ]];then echo $a;fi; done \
     |while read b;do echo $((b*2));done
    1
    2
    3
    4
    5
    eof

(Better expressed with seq):

    seq 5 \
    | while read a;do if [[ $a -gt 2 ]];then echo $a;fi; done \
    | while read b;do echo $((b*2));done

But we can do (a little) better:

    # Ok, this is a bit odd, the filter is actually inverse...
    filter()
    {
        while read it
        do
          #NOT SAFE!!
          if eval ${*}
          then
            echo "${it}"
          fi
        done
    }

    map()
    {
        while read it
        do
          #NOT SAFE!!
          eval ${*}
          echo "${it}"
        done
    }

    #Valid bash:
    seq 5 \
    | filter test '$it' -gt 2 \
    | map 'it=$((it*2))'

Sorry, didn't mean to turn this into stack overflow, the "simple" syntax just tickled me...

Some languages have a sort-of default variable name for cases like this, namely "it":

> put 1 2 3 4 5 | filter > $it 2 | map * 2 $it

As zokier was presumably pointing out, other languages have a different sort-of-default variable name, namely `$_` (http://c2.com/cgi/wiki?DollarUnderscore). :-)