docs: clarify D directive's influence on display settings once and for all

MANUAL.md

@@ -170,7 +170,7 @@ Account names may contain single spaces, eg: `assets:accounts receivable`.
 
 ### Amounts
 
-After the account name, separated by *two or more spaces*, there is
+After the account name, separated by ***two or more*** spaces, there is
 usually an amount.  This is a number, optionally with a currency symbol or
 commodity name on either the left or right. Commodity names which contain
 more than just letters should be enclosed in double quotes.
@@ -178,16 +178,31 @@ more than just letters should be enclosed in double quotes.
 Negative amounts usually have the minus sign next to the number: `$-1`.
 Or it may go before the symbol: `-$1`.
 
-The number may optionally have a decimal point, either a period (`.`) or a
+hledger supports flexible decimal points and digit group separators so you
-comma (`,`). hledger's reports will generally use the highest precision
+can use your country's convention.  Numbers can use either a period (`.`)
-you have used in each commodity.
+or a comma (`,`) as decimal point. They can also have digit group
+separators at any position (eg thousands separators) which can be comma or
+period - whichever one you did not use as a decimal point. If you use
+digit group separators, you must also include a decimal point in at least
+one number in the same commodity, so that hledger knows which character is
+which. Eg, write `$1,000.00` or `$1.000,00`.
 
-Numbers may also have digit group separators, eg thousands separators.
+### Commodity display settings
-hledger's reports will follow the digit groups you have used.  The
+
-separator character is either comma or period - whichever one you did not
+Based on how you format amounts, hledger will infer canonical display
-use as a decimal point. If using digit group separators you should write
+settings for each commodity, and use them consistently when displaying
-at least one number with a decimal point, so hledger will know which is
+amounts in that commodity. These settings include:
-which. Eg: `1,000.00` or `1.000,00`.
+
+- the position and spacing of the currency/commodity symbol
+- the digit group separator character and digit group sizes, if any
+- the decimal point character
+- the number of decimal places
+
+The canonical display settings are generally those used in the first
+amount seen, and the number of decimal places is the highest used in all
+amounts, in the given commmodity.
+[Default commodity directives](#default-commodity) can also influence the
+canonical display settings.
 
 ### Simple dates
 
@@ -248,10 +263,8 @@ Example:
 
 ### Default commodity
 
-You can set a default commodity or currency with a D directive. The
+You can set a default commodity or currency with a D directive. This will
-commodity will be used for any subsequent amounts which have no commodity
+be used for any subsequent amounts which have no commodity symbol.
-symbol. This directive also influences the display format for amounts in
-that commodity.
 
     ; default commodity: british pound, comma thousands separator, two decimal places
     D £1,000.00
@@ -260,6 +273,10 @@ that commodity.
       a  2340   ; no commodity symbol, will use the above
       b
 
+If such an amount is the first seen in that commodity, the canonical
+[commodity display settings](#commodity-display-settings) will also be
+taken from the directive.
+
 ### Prices
 
 You can specify a commodity's unit price or exchange rate, in terms of

tests/default-commodity.test

@@ -1,39 +1,86 @@
 # a default commodity defined with the D directive will be used for any
 # commodity-less amounts in subsequent transactions.
-#
+
+# 1. no default commodity
 bin/hledger -f- print
 <<<
-
+2010/1/1
-; no default commodity
-2010/1/1 x
   a  1000
   b
-
-; pound, two decimal places, no digit group separator
-D £1000.00
-
-2010/1/1 y
-  a  1000
-  b
-
-; dollar, comma decimal point, three decimal places, no digit group separator
-D $1,000
-
-2010/1/1 z
-  a  1000
-  b
-
 >>>
-2010/01/01 x
+2010/01/01
     a          1000
     b         -1000
 
-2010/01/01 y
+>>>=0
+
+# 2. pound, two decimal places, no digit group separator
+bin/hledger -f- print
+<<<
+D £1000.00
+2010/1/1
+  a  1000
+  b
+>>>
+2010/01/01
     a      £1000.00
     b     £-1000.00
 
-2010/01/01 z
+>>>=0
+
+# 3. dollar, comma decimal point, three decimal places, no digit group separator
+bin/hledger -f- print
+<<<
+D $1,000
+2010/1/1
+  a  1000
+  b
+>>>
+2010/01/01
     a     $1000,000
     b    $-1000,000
 
 >>>=0
+
+# 4. dollar, three digit group separator, one decimal place
+bin/hledger -f- print
+<<<
+D $1,000.0
+2010/1/1
+  (a)  1000000
+>>>
+2010/01/01
+    (a)  $1,000,000.0
+
+>>>=0
+
+# 5. as above, sets the commodity of the commodityless amount, but an
+# earlier explicit dollar amount sets the display settings for dollar
+bin/hledger -f- print
+<<<
+D $1,000.0
+2010/1/1
+  (a)  $1000000.00
+  (b)   1000000
+>>>
+2010/01/01
+    (a)   $1000000.00
+    (b)   $1000000.00
+
+>>>=0
+
+# 6. as above, but the commodityless amount is earliest, so it sets the
+# display settings for dollar. The greatest precision is preserved though.
+bin/hledger -f- print
+<<<
+D $1,000.0
+2010/1/1
+  (a)   1000000
+  (b)  $1000000.00
+>>>
+2010/01/01
+    (a)  $1,000,000.00
+    (b)  $1,000,000.00
+
+>>>=0
+