Sorry, I discovered the issue myself

Space between the : and = sign was the origin of error.

Problem solved by itself

> 25 feb. 2020 kl. 18:34 skrev Jan-Erik Hägglöf <janerik.hagglof@bahnhof.se>:
> 
> The following code gives an error, my goal is to draw 4 different arrows but only three functions successfully.
> 
> The error log shows
> 
> metapost        > initializing number mode 'scaled'
> metapost log    > 
> metapost log    > loading metafun, including plain.mp version 1.004 for metafun iv and xl
> metapost log    > 
> metapost log    > >> unknown path D
> metapost log    > ! Isolated expression.
> metapost log    > <to be read again> 
> metapost log    >                    :
> metapost log    > <*> ... :=(12cm, -1cm)--(11cm, -1cm); path D ; D :
> metapost log    >                                                    =(4cm, -1cm)--(3cm, -1cm)...
> metapost log    >
> 
> And the code is as follows
> 
> \startMPcode 
> path A ; A :=(0cm, -1cm)--(1cm, -1cm);
> path B ; B :=(13.5cm, -1cm)--(14.5cm, -1cm);
> path C ; C :=(12cm, -1cm)--(11cm, -1cm);
> path D ; D : =(4cm, -1cm)--(3cm, -1cm);
> drawarrow A; drawarrow B; drawarrow C ; drawarrow D ;
> \stopMPcode
> 
> Unless I uncomment D it gives the above error.
> 
> Mysterious issue
> 
> /Janne
> ___________________________________________________________________________________
> If your question is of interest to others as well, please add an entry to the Wiki!
> 
> maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
> webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
> archive  : https://bitbucket.org/phg/context-mirror/commits/
> wiki     : http://contextgarden.net
> ___________________________________________________________________________________